1. Explain the term Aliquot and Diluent?
Ans:
- Aliquot: It is a measured sub-volume of the original sample.
- Diluent: Material with which sample is diluted
2. What is molality?
Ans: Molality is the number of solutes that are present in 1 kg of a solvent.
3. What is titration?
Ans: Titration is a process to determine the molarity of a base or an acid. In this process, a reaction is carried out between the known volumes of a solution with a known concentration, against the known volume of a solution with an unknown concentration.
4. What is a buffer?
Ans: A buffer is an aqueous solution that has a highly stable pH. It is a blend of a weak acid and its conjugate base or vice versa. On adding a small amount of base or acid to the buffer, its pH hardly changes.
5. How does buffer work?
Ans: In buffer when hydrogen ion is added, it will be neutralized by the base in the buffer. Hydroxide ion will be neutralized by the acid. On the overall pH of the buffer solution, these neutralization reactions will not show much effect.
While when you select an acid as a buffer solution, try to use an acid that has a pH close to your desired pH. This will help your buffer to achieve a nearly equivalent amount of acid and conjugate base, so that it will enable you to neutralize as much as H+ and OH -.
6. What is a mole?
Ans: Mole is the unit used to define the number of chemical substances present in a substance. It is the amount of substance which consists of the same number of chemical units as there are atoms in exactly 12 gram of pure carbon-12.
7. How will you calculate how many moles of glucose present in 320 mL of 5.0 M of glucose solution?
Ans: First step: Convert the volume from millilitres to litres
- 320 X (1 litre/1000mL) = 0.320 L solution
Second use the formula = M x V
= 5.0 moles glucose/ litre solution X 0.320 L solution
= 1.6 moles of glucose present in 320mL of solution
8. Explain what is the difference between fractionation and distillation?
Ans: Both methods are used to separate the components present in the solution based on the melting points
- Distillation: This technique is used when the boiling point of chemicals are different in the mixtures
- Fractionation: This technique is used when the boiling point of chemicals are close to each other in the mixtures
9. Mention the formula to calculate the pH of a solution?
Ans: In order to calculate the pH of a solution you have to use the formula pH= -log [H+] or pH = -log [H3O+]
10. What is the difference between Molarity and Normality?
Ans: Both techniques are used to the number of chemicals present in the solution. However, they are almost similar but differs in
| Molarity | Normality |
| Molarity is used to know the total amount of molecules in a 1-liter solution | Normality is used to know the total number of reactive units in 1 liter of solution |
| It is expressed as moles of a compound per liter of solution | t is expressed in equivalent per liter |
11. What is Valency?
Ans: Valency is a property of groups or atoms, equal to the number of atoms of hydrogen that the group or atom could combine with or displace in forming compounds.
12. Define Avogadro’s law?
Ans: According to Avogadro’s law, at the same temperature and pressure, an equal volume of gases contains the same number of molecules regardless of the chemical nature and physical properties.
Avogadro’s number = 6.023 X 10 (-23)
13. Explain what makes a molecule into an organic molecule?
Ans: In a molecule when a hydrogen atom is less than the ratio of the carbon atom, then such molecules are referred to as an organic molecule.
14. Which metal is used to extract copper from the solution of copper sulfate
Ans: Fe or ferrous is the metal that is used to extract copper from the solution of copper sulfate.
15. Explain what is the chemical composition of fat in the human body?
Ans: The fat found in the human body is mainly composed of
- Glycerides
- Glycerides+Phospholipids
- Glycolipids
- Phosphoinositides
- Tocopherol
16. What is the monomer of polyethylene?
Ans: The monomer of polyethylene is ethylene
17. What is the formula you will use to calculate how many milliliters of 5.5 M NaOH are required to prepare 400 mL of 1.5M NaOH?
Ans: To know the amount or volume of NaOH to prepare 400 mL of 1.5 M NaOH, we use the formula
M1 X V1 = M2 X V2
V1 = M2 X V2/ M1
But before that, we will convert 400 mL into liter = 0.4 L
5.5 X V1 = 1.5 M x 0.4 L
V1 = 1.5 M X 0.4L/ 5.5
V1= 0.10 L
V1 = 100mL
So, you need 100mL of 5.5 NaOH
18. Why graphite rod is used in nuclear reactors?
Ans: A graphite rod is used in a nuclear reactor allows un-enriched uranium to be used as a nuclear fuel.
19. Mention how many milliliters are equal to 1 liter and how many microliters are equal to liter?
Ans:
- 1 millilitre = 0.001 litre
- 1 microliter = 0.000001 litre
20. What is oxidation and reduction reaction?
Ans:
- Oxidation = When there is a loss of hydrogen or electrons OR gain of oxygen is known as Oxidation reaction.
- Reduction = When there is a gain of hydrogen or electron OR loss of oxygen is known as a reduction reaction
An example of an oxidation-reduction reaction is observed in the human body when an electron is transferred into the cell and oxidation of glucose takes place from which we get the energy.
21. What does iron ore consist of?
Ans: Iron ore is consists of Fe2O3
22. Explain what is dextro-rotatory and levorotatory?
Ans: Levorotation and Dextrorotation are referred to as the properties of plane-polarized light when light rotates clockwise when it approaches the observer is then known as dextro-rotation, and when the light rotates anti-clockwise then it is referred to as levorotation.
A compound that exhibits a dextro-rotation is referred to as dextro-rotatory and which exhibits levorotation is referred to as levorotatory.
23. UV, RI, PDA detector principles?
Ans: The UV, RI, PDA detector principles are:
Ultraviolet (UV) :
- Electromagnetic radiation with a wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays.
Refractive index :
- The refractive index, also known as the index of refraction, is defined as the ratio of the velocity of light in a vacuum (or air) to its velocity in a transparent medium.
PDA – Photo Diode :
- A photodiode is a semiconductor device that converts light into an electrical current. The current is generated when photons are absorbed in the photodiode. Photodiodes may contain optical filters, built-in lenses, and may have large or small surface areas.
24. Why Na lamp is used in a polarimeter?
Ans: Sodium (Na) Lamp is used in a polarimeter because it gives monochromatic light and also gives high-energy output. High sensitivity and S/N ratio.
Line spectrum i.e. gives monochromatic light of 589nm.
A polarimeter is a scientific instrument used to measure the angle of rotation caused by passing polarized light through an optically active substance.
25. What is a unit of KF?
Ans: KF is the molal freezing point depression constant of the solvent.
For factor determination: mg/ml(milligram per milliliter).
For water content
For solids =%w/w
For Liquid =%w/v
Note
mL of KF reagent is equal to 5mg of water.
26. What is organic chemistry?
Ans: Organic Chemistry is the study of the structures, properties, and behaviors of compounds containing carbon-hydrogen bonds.
Organic compounds are molecules composed of carbon and hydrogen and may contain any number of other elements.
Many organic compounds contain nitrogen, oxygen, halogens, phosphorus, silicon, and sulfur.
27. Why Water and Sodium Titrate only uses for the Calibration of Karl Fischer Titrator?
Ans: Sodium tartrate dihydrate is the volumetric standard for Karl Fischer titration. Under normal conditions, it is stable and non-hygroscopic.
Sodium tartrate dihydrate has a stoichiometric water content of 15.66% and is mainly used for titer determination in volumetric.
Karl Fisher Titration
Karl Fisher Titration is a method for the determination of moisture content. The technique was established by a chemist named Karl Fischer. It is based on a mixture that reacts with water and converts the water into a non-conductive chemical. Karl Fisher provides for the specific detection of water. There are two methods used to perform the Karl Fischer titration test.
- Volumetric Karl Fischer
- Coulometric Karl Fischer
Volumetric Karl Fischer
In this method, the moisture determination is based on the amount, or volume, of reagent used to convert the water.
Samples are dissolved in a solvent before the titration begins.
A reagent is added until the water is removed.
Coulometric Karl Fischer
In this method, the reagent and solvent are combined in the titration cell.
When a sample is introduced into the titration cell and dissolved, the reagent is released by the induction of an electrical current.
28. What is the difference between residue on ignition and sulfated ash?
Ans: Residue on ignition means the residue that leftover on the ignition, the residue may be of any kind i.e. sulfate, or TiO2, or some other metal contamination. Sulfated ash means the ash should be of the sulfate counterpart.
The Residue on Ignition / Sulfated Ash test utilizes a procedure to measure the amount of residual substance not volatilized from a sample when the sample is ignited in the presence of sulfuric acid according to the process.
This takes a look that is typically used to determinative the content of inorganic impurities in AN organic substance.
29. Why ph is between 0 to 14?
Ans: The pH scale is maintained to range from 0-14 or maybe 1-14, but neither is correct.
The pH range does not have an upper nor lower bound.
The pH is an indication of the concentration of H+.
For example, at a pH of zero the hydronium ion concentration is one molar, while at pH 14 the hydroxide ion concentration is one molar.
Typically, the concentrations of H+ in water in most solutions fall between a range of 1 M (pH=0) and 10-14 M (pH=14).
Hence a range of 0 to 14 provides sensible (but not absolute) “bookends” for the scale.
30. Why we are using polystyrene film for calibration of IR spectroscopy?
Ans: Polystyrene film is made by the polymer of styrene. It’s having highly durable and stable at any temperature because it’s made of polymers.
Polystyrene films have long been used for checking the final performance of infrared (IR) spectrometers and specific tests for wavenumber accuracy and determination supported such films square measure protected in varied pharmacopeias (1–4).
31. What is the difference between calibration and validation?
Ans:
Calibration | Validation |
| Calibration is a process that ensures that accuracy is maintained in the measurements produced by your equipment. | Validation is a documented process that provides assurance that a product, service, or system consistently provides results within the acceptable criteria. |
| The calibration performance of any equipment is compared against a reference standard. | There are no reference standards used in the validation. |
| Calibration assures accuracy of measurements. | Validation provides proof of consistency across all the processes, batches of products, or methods being used. |
| You must periodically calibrate your instruments. Identify if there is a ‘drift’ in the measurements and eliminate it through calibration. | There are no such requirements for validation. It should be performed when you make any change in the existing system or when the revalidation period has reached. |
| It should be performed as per calibration SOP. | It should be performed as per the validation protocol. |
32. What is the difference between paddles and Basket? Why we are using a Basket or paddle in dissolution?
Ans: Paddle and baskets are the different types of apparatus used in dissolution to find the drug release.
Generally, baskets are used for capsules or tablets floating in media. the paddle can be used for all.
Sometimes paddle is also used in the capsule and floating tablets by using a sinker to achieve the appropriate results.
Paddle Dissolution Method:
In this method, the rotating shaft is fixed to a blade attached vertically at the end.
This blade is meant to act as a stirrer to mix the drug being tested with the liquid inside the holding vessel.
Typically, the drug is placed inside the holding vessel and once it settles at the bottom, the rotating shaft is switched on to begin mixing.
The drug being tested is also frequently attached to a sinker to prevent the drug from attaching itself to the vessel walls and also to ensure the drug being tested remains positioned beneath the rotating shaft.
This sinker is made from a non-reactive element to prevent it from reacting with the drug sample it holds.
Basket Dissolution Method
In the basket dissolution method, a different apparatus is placed at the end of the rotating shaft. This apparatus is called a basket.
It is cylindrical in shape and is constructed from non-reactive mesh to prevent any unwanted chemical reactions that may alter the final result.
The pores in the mesh allow the dissolving drug to move from the basket into the holding vessel, creating a uniform solution.
The drug being tested is first placed inside the basket that is firmly attached to the end of the rotating shaft.
Once the drug is securely in place, the motor is switched on and the shaft begins rotating the basket inside the vessel.
The sticky tablets containing HPMC or similar compounds as a binder are tested using a basket because the tablet swells in water and may stick to the walls of the dissolution vessel.
33. Which of the following is a non-metal that remains liquid at room temperature?
Bromine
Chlorine
Helium
Phosphorus
Ans: 1. Bromine
Bromine is a synthetic component with the symbol Br and nuclear number 35.
It is the third-lightest halogen and is a fuming red-brown fluid at room temperature that dissipates promptly to frame a comparably shaded gas.
These properties are intermediate between those of chlorine and iodine.
ORGANIC CHEMISTRY INTERVIEW QUESTIONS WITH COMPLETE ANSWERS
34. Trimethylamine Is A Pyramidal Molecule. What Is The Hybridization At Nitrogen?
Ans: The nitrogen is bonded to three CH3 groups and has a lone pair, so the hybridization is sp3.
35. Which One Of The Following Is Not Aromatic? 1)cyclotetrane 2)benzene 3)napthalene 4)anthracene ?
Ans: cyclotetrane, because the double bonds are not conjugation
36. Number Of Sigma Bonds Present In Benzene Are Same As ..?
Ans: The number of Sigma bonds present in Benzene is the same as Borazine (inorganic Benzene).
no. of sigma bonds in Benzene= 12 (C6H6)
no. of sigma bonds in Borazine= 12 (B3N3H6)
37. The Name Benzene Was Proposed By?
Ans: Kekule
38. Which Is Useful In The Manufacture Of Insecticides?
Ans: Rotenone, Azadirachtin are natural insecticides, synthetic routes are developed recently.
DDT, Gammaxene were used but were scraped out as they pose health hazards and are insects are able to sustain the max. dosages every year. Generally, Semiochemicals, a class of compounds found in insects themselves are being studied and used as effective insecticides and warfare agents.
39. What Are The 4 Classes Of Unsaturated Fatty Acids?
Ans: Monounsaturated fat: This is a type of fat found in a variety of foods and oils.
Polyunsaturated fat: This is a type of fat found in plant-based foods and oils. They can be divided into two groups known as omega-3 fats and omega-6 fats.
Omega-3 fatty acids: One type of polyunsaturated fat made up of mainly omega-3 fatty acids and may be beneficial to the heart.
Omega-6 fats decrease the risk of heart disease when they are consumed in place of saturated and trans fats.
40. What Is The Difference Between Coordinate Covalent Bond And Covalent Bond?
Ans: In coordinate covalent bond electrons which are shared by both the atoms are contributed by only one atom. This is otherwise known as the dative bond.
In a covalent bond, the electrons shared by both the atoms are contributed by both the atoms equally.
41. What Happens When Aminoacids React With Alcohols?
Ans: when alcohols react with amino acids esters are formed. The esters are volatile in contrast to free amino acids.
R-CH-COOH + H2O-C2H5 -> R-CH-COO-C2H5 + H2O
NH2 NH2
42. What Are Ligases? which type of reactions they will catalyze?
Ans: Ligases are the group of enzymes that catalyze the transfer of a group from one kind of molecule to another. These are also called transferases.
43. NH2-CH2-COOH Belongs To What Structure?
Ans: its the structure of amino acid glycine
44. Mention The Factors Affecting Solubility Of Compounds?
Ans: temperature, pH, the structure of compound i.e., whether the compound is straight-chain or branched & molecular weight of the compound.
45. What Are Geometrical Isomerism? Explain Them With An Example?
Ans: Geometrical isomerism is an example of stereo-isomerism.
This occurs when substances have the same molecular formula, but a different arrangement of their atoms in space. There are three ways that this can happen:
where there is a C=C bond in the molecule;
where a molecule has rings; or
where there is a >C=N bond.
46. How ‘salting Out’ Process Is Used In Extraction?
Ans: Proteins are precipitated from an aqueous solution by high concentrations of neutral salts. This is the”salting-out” process.
Divalent and trivalent ions are more effective than univalent ions.
The salts commonly used for this purpose are NA2SO4,(NH4) 2SO4 Magnesium salts, and phosphates.
47. What Is Saponification Of Oils?
Ans: When fats are hydrolyzed with alkali, the free fatty acids react with alkali to form metallic acids, these salts are soaps and this process is called saponification.
48. What Is Redox Potential Of An Organic Compound?
Ans: The quantitative measure of the affinity of a compound to lose or gain electrons is the redox potential.
49. What Is Lithipone?
Ans: Lithipone is a white pigment.it is a mixture of BaSO4&ZnS.
50. What Is the Difference Between Anti-aromatic And Non-aromatic Compounds?
Ans: Anti aromatic compounds follow 4npi electron rule and are cyclic compounds, planar, whereas nonaromatic is non-planar or planar, cyclic or acyclic.
51. What Is The Range Of Uv & Vis Lamp In Spectrophotometer?
Ans: UV lamp range is 200 nm to 340 nm.
vis lamp range is 320 nm to 1100 nm.
both lamp range is mfg instrument company voice of record is could the data range in the lamp is 200 nm to 1200 nm.
52. If Benzene And Toluene Both Can Be Mixed… What Is The Boiling Point For The Mixture And Its Density?
Ans: If benzene and toluene mixed the boiling point will be ~ 90oC. Density will not change.
53. Benzoic Acid Is A Weaker Than Para Nitro Benzoic Acid?
Ans: In p-nitrobenzoic acid due to -I effect of nitro group hydrogen present in acid becomes more electronegative and easily remove as proton and acts as a strong acid.
54. What Is The Difference Between Sulphated Ash And Roi?
Ans: There no difference between procedures, both are the same. we generally term it as sulfated ash test/ROI.in Indian pharmacopeia, called it ROI and in USP, called it as sulfated ash tesh. both are used for the determination of the content of inorganic matter in an organic substance.
55. What Is The Difference Between Pvc And Upvc Plastic?
Ans: PVC-POLYVINYL CHLORIDE AND uPVC IS THE Unplasticized polyvinyl chloride IT IS USED FOR the building industry as a low-maintenance material.
56. What Mean By Organic Chemistry?
Ans: Organic Chemistry is a branch of Science, which deals with the study of carbon compounds except for Carbon monoxide, Carbon dioxide, and carbonates.
57. Which Organic Compound Is Used As A Fuel In Jet Air Crafts? A. 100 Octane B. 90 Octane C. Kerosine Oil D. Diesel Oil
Ans: Kerosine oil
58. What Is The Difference Between An Ion And A Radical?
Ans: An ion is a species that has no unpaired electron and is charged, either positively or negatively i.e. maybe a cation or an anion. It is formed due to the heterolysis of a molecule
e.g HCl—-> H+ + Cl-
A radical is a neutral species having at least one unpaired electron and is formed due to the homolysis of a molecule
e.g H-H ——-> H* + H*
The * shows an unpaired electron. Similarly CH3* is radical.
59. How To Synthesis Toluene To Benzaldehyde?
Ans: from direct reaction of toluene to crotyl chloride (CrO2Cl2) at room temp. to form benzaldehyde.
60. How To Make A 100ml Solution With Given 10gm Of NaOH?
Ans: Take 100ml water and pore 10gr NaOH pellets, stir with gently boiling. NaOH solution is ready.
61. How We Can Differentiate Between Oxalic Acid And Tartaric Acid?
Ans: By Fluorescein test … where tartaric acid ” used in baking powder and found in many food products
oxalic acid: harmful in contact with skin or eyes toxic .remove Ca ions From blood and also block Kidneys Fluorescein test depended on Light passes & usually in UV light
Analytical Chemistry Interview Questions With Answers
62.. What Is Standard Deviation?
Ans: Standard deviation tells about the widely spread out of the measured analytical data points. Highly precise measurements are less standard deviation.
63. How To Calculate Signal To Noise Ratio By This Formula 2h/h Where H From Where And How To Measure?
Ans: The “H” is the height of the main peak and the “h” is the lowest height of a peak in the range of X5 the main peak (around the main peak).
64. What Is the Use Of Ion Pair Reagents?
Ans: The chemical substances that pair each other, form complexes. These can use for stabilization of one of the molecules that are more active or for colourifying etc.
65. Explain What Is A Base Line?
Ans: The baseline is nothing but the response of the detector to the mobile phase.(GC,HPLC)
The baseline should be stable to start a run.
Un stabilized baseline is called baseline noise.
Baseline noise is of many types.
Cyclic baseline
Synchorinise noise
Asynchorinise noise
Baseline drift
Spikes
Negative peaks
No peaks
Baseline noise is attributed due to the mobile phase, sample, system problems, leaks in the system and temperature variations in the lab, etc.
66. What Is Deference Between The Working Standard And Reference Standard?
Ans: Reference std is like USP, EP std, in this std we get COA including results of Potency by HPLC, NMR & XRPD data. whenever there is the requirement for w.std preparation first we have to get API raw material from the warehouse & we check assay, water content, or LOD & Related substances against reference std. Sometimes we are doing analysis in duplicate & get the mean value, then we decide the final assay or potency value & its validity one year
67. How Do We Fix The Sample Concentaryion In Hplc Method Development? What Is The Basis?
Ans: If all impurities at spec level are meeting s/n ratio more than 60 at that particular concentration as well as no column overload.
68. Suppose We Prepared Ph Buffer Solution In Lab. and Calibrate Against Nist Solution, And Same Shall Going To Used Up To 30 Days. Is It Possible To Stable Ph And Appearance? Suggest?
Ans: PH of the buffer solution will be changed, due to the presence of nitrogen-containing substances and also due to microbial growth. but if stored at 4-degree centigrade. it may use to some extent.
69. What Is Rs Test Why We Are Performing Rs Test?
Ans: RS stands for related substance, means by-products upon completion of the reaction, or the unreacted portion present in the product. RS testing helps us to identify the adequate quantity of these by-products or unreacted parts. Moreover, RS help us during the stability study to know any significant change in the nature of the product during long storage in various climatic conditions
70. What Is the Difference Between Silica Used In Tlc And HPLC Column?
Ans: Tlc is called thin-layer chromatography. The simple technique works on the principle of adsorption, whereas HPLC is a sophisticated technique that works on the partition principle.
71. What Is Heavy Water?
Ans: Isotopes of H2O, D2O.
72. How To Know Hplc Column Performance?
Ans: Check for the number of plates that it maintains steady all over the run, check the SST at every run that complies with the acceptance criteria of the method.
73. What Is Use Of Acetonitrile Compare To Methanol In Rp-HPLC Method Development?
Ans: ACN is highly polar as compare to Methanol So provide Better resolution for many compounds and it has the property to form hydrogen bond so provide better selectivity.
74. What Is Quality Control?
Ans: Quality control means maintaining the quality of the product by calculating their content, different physical parameters, as per their specification IP/BP/USP/EP/JP.
75. When We Get Moisture Content By Kf Higher Than By Lod, What Does This Indicate?
Ans: KF titration is the most accurate method in analyzing moisture content. But it is costlier and maintenance is too high. So in the analysis of moisture content, an alternative experiment is Loss on Drying(LOD), which gives approximate moisture value present in the given substance.
76. Will You Dry Dust Before Factor Determination By Using Karl Fisher Titrator?
Ans: No, We will not dry dust before factor determination by using Karl fisher titrator.
77. In Rs Method Development When We Are Going Area Normalization Method To Dilute Standard Method?
Ans: When the analyte gives a linear response then only we can use area normalization. When analyte response is too high and saturated in chromatograph, for this condition we can use diluted standard.
78. In The Gas Chromatography What Is Used Carrier Gas And Makeup Gas?
Ans: In gas chromatography, nitrogen gas is used as carrier gas and makes up gas.
79. What Is The Difference Between C8 And C18 HPLC Column?
Ans: C8 column is more polar than C18, and it has 8 carbon atoms.
80. Why Karl Fisher Factor Is 5?
Ans: 5 mg of water-consuming 1 ml of KF reagent.
81. In Ph Calibration, calibration Is Done For Ph Meter Or Electrode? how Will You Judge That The Meter To Be Calibrated With 4,7,9.2 Buffers? explain Temperature Effects On Ph?
Ans: Calibration is also one of the system suitability tests, whether the system working satisfactory or not physically so when you do the calibration for pH meter with standard pH buffer solution (which is from NIST-it may accurate solution for actual pH)and once if your instrument has calibrated, you can cross-verify your instrument against those pH buffer solution (NIST) and the acceptable range is +/- 0.02 then you can conclude for. once if the temperature raises the pH will decrease and it’s not all cases.
82. What Is Delay Volume?
Ans: The volume from gpv to detector.
83. Why Six Units Used For Precision?
Ans: Since it is performed as per ICH guidelines for validation.
84. Loss Of Linearity In Tkn Analysis, Possible Reasons?
Ans: Contamination in calibration standards during the TKN digestion.
85. What Is The Use Of Tlc And Hplc? And When And Where Use?
Ans: TLC is used to check whether the reaction is completed or not. HPLC is used to identify, quantify, and for separation of a mixture
Polymer Chemistry Interview Questions With Answers
86. What Are Some Of The Polymers That You Encounter Every Day? Describe Their Physical Properties.
Ans: Sandwich bags, carpets, nylon stockings, stackable chains, milk cartons, etc.
87. Why Do Different Polymers Have Different Properties?
Ans: They have different chemical compositions (different monomer units), different structures, different ways of being fabricated, etc.
88. Why Are Olefins (alkenes) Good Monomers For Polymerization Reactions?
Ans: The electrons in the weak p-bonds can be used to form strong s bonds to other monomer units.
89. What Kinds Of Structural Changes Accompany Bond-breaking And Bond-forming In Olefin Polymerization?
Ans: The olefin monomers are flat (two-dimensional) molecules with sp2-hybridized carbon atoms. The polymers are three-dimensional molecules in which the carbon atoms are sp3 hybridized.
90. Examine Samples Of Ldpe (sandwich Bag, Squeeze Bottle) And Hdpe (milk Jug, Grocery Bag). What Are Some Of The Differences In The Physical Properties Of These Substances?
Ans: LDPE – more transparent, flexible, waxy.
HDPE – more opaque, rigid, non-waxy.
91. How Does The Molecular-level Structure Of These Polymers Influence Their Physical Properties?
Ans: The structure (e.g., extent of branching) determines how the individual polymer molecules can orient (or “pack”) in the solid state. This, in turn, influences physical properties such as density, crystallinity, melting point, and strength.
92. How Can Chemists Control Which Type Of Polyethylene (LDPE Vs. Hdpe) Is Generated?
Ans: Through the choice of appropriate catalysts and reaction conditions.
93. Besides The Extent Of Branching, Can You Think Of Any Other Structural Parameters That Might Lead To Differences In Physical Properties?
Ans: The average value of n (the number of monomer units in the polymer) and the range in individual values of n.
94. Does Ethylene Polymerize Under Mild Conditions In The Absence Of A Catalyst?
Ans: No, in the absence of a catalyst, ethylene molecules would need to collide at very high energy in order to react with each other.
95. What Is The Role Of A Catalyst?
Ans: A catalyst reduces the energy of activation for a reaction by providing an alternative pathway. In this way, it speeds up the reaction and allows it to proceed under milder conditions.
96. Why Are Metals Often Good Catalysts?
Ans: They provide a site where organic molecules can come together and react.
97. What Is The Difference Between A Heterogeneous And A Homogeneous Catalyst? What Are Some Of The Advantages Of Homogeneous Catalysts?
Ans: Heterogeneous catalysts are insoluble in the reaction medium, while homogeneous catalysts are soluble. Since homogeneous catalysts are generally molecular species, they are more amenable to study using the spectroscopic tools of chemistry. In addition, they can be chemically modified or “tailored” to produce polymers with a particular kind of structure.
98. How Would You Describe The Orientation Of The Ligands Around The Zr Center In The Homogeneous Zirconium Catalyst?
Ans: The ligands – the two cp’s, the alkyl group, and the olefin (or open site) – are oriented in a tetrahedral fashion around Zr.
99. What Is The Nature Of The Bonding Interaction Between A Metal And An Olefin?
Ans: The olefin uses the electrons in its p-bond to interact with the metal.
100. Polymer Chain Growth Can Be Terminated By B-hydride Elimination Or By Reaction With H2. What Is One Advantage Of The H2 Reaction?
Ans: It allows the chemist to stop chain growth at the desired stage, rather than relying on the “natural” process of b-hydride elimination. Hence, it gives the chemist some control over the value of n.
101. What Are The Other Ways In Which Chemists Can Manipulate The Properties Of Polymers?
Ans: By using different monomers. For example, the incorporation of a phenyl (C6H5) unit into the monomer leads to polystyrene, while the incorporation of a chloro (Cl) group leads to PVC, a polymer with very different properties.
By using different fabrication techniques. For example, polystyrene can be glassy or foamy depending on how it is fabricated.
102. “co-polymers” Consist Of Two Different Monomers (“a” And “b”) Joined In An Alternating Fashion (ababab…). Block Copolymers Also Consist Of Two Different Monomers, But In This Case Blocks Of Polymer Containing, Only A Units Are Joined To Blocks Of Polymer Containing Only B Units (aaaaabbbbb…). How Might Block Copolymers Be Synthesized?
Ans: After the polymerization has been allowed to proceed with monomer A, the olefin feedstock is changed to B and the polymerization continues.
103. A Polymer’s Structure Influences Its Physical Properties. Describe Two Structural Variations That Are Possible For Polypropylene But Not For Polyethylene.
Ans: The orientation of the monomer units along the chain (head-to-tail, head-to-head, random) and the orientation of the methyl groups with respect to the polymer backbone (tacticity).
104. Consider The Polymerization Of Vinylidene Chloride, Ch2=ccl2. What Structural Variations Are Possible In Poly(vinylidene Chloride)?
Ans: Orientation of the monomer units along the chain.
105. Head-to-tail Polymerization Of Propylene Is Observed With The [cp2zr(r)]+Catalyst. Explain This Result On The Basis Of Molecular-level Interactions.
Ans: Each incoming propylene molecular orients with its methyl group in toward R, rather than out toward cp, in order to avoid unfavorable contacts with the bulky cp’s. When the R group migrates to propylene, it migrates to the closer olefinic carbon, which is always the one bearing the methyl group (the “b carbon”).
106. Atactic Polypropylene Is Always Produced With The [cp2zr(r)]+Catalyst. Explain This On The Basis Of Molecular-level Interactions.
Ans: There is no preference for the methyl group on propylene to be oriented up or down because in each case it has exactly the same interaction with a cp group. Since there is no up/down preference, a random (atactic) orientation of methyls along with the chain results.
107. What Does It Mean For A Molecule Or A Ligand To Be “chiral”? What Properties Does Chirality Impart To A Molecule?
Ans: Molecules that are not superimposable on their mirror images are chiral. Mirror image isomers are called enantiomers. Enantiomers have identical physical properties except that they rotate plane-polarized light in opposite directions.
108. Explain What Is Meant By “c2 Symmetry” And “mirror Plane Symmetry”. Can A Molecule With “mirror Plane Symmetry” Ever Be Chiral?
Ans: “C2 symmetry” means that 180o rotation about an axis through the molecule results in a geometry equivalent to the starting geometry. “Mirror plane symmetry” means that one half of the molecule can be perfectly reflected into the other half through a symmetry plane. Molecules with mirror plane symmetry cannot be chiral.
109. Consider A Homogeneous Zirconium Catalyst In Which Two Cyclopentadienyl Ligands Are Connected By A —ch2–ch2— Bridge. Which Symmetry Elements Does The (bridged-cp2)Zr Moiety Possess? What Is The Expected Tacticity Of The Polypropylene Produced Using This Type Of Catalyst? Why?
Ans: The (bridged-cp2)Zr moiety possesses both C2Zr catalysts discussed in Session 5, it would be expected to produce atactic polypropylene, since the methyl group on the propylene would have no up/down prepreference.
