JEE Chemistry Polymers MCQs Set 05

Answer: (b) a, b and c

Explanation:
1. Bakelite is made by condensing phenol with formaldehyde, so (a) is correct.
2. Its chains are joined by covalent cross links, so (b) is correct.
3. Cross linked polymers cannot be softened again on heating, so it is thermosetting and (c) is correct.
4. It is synthetic, so (d) is wrong. The correct choice is a, b and c.

Teacher's Note:
1. Bakelite is the standard example of a thermosetting resin.
2. Remember: phenol plus formaldehyde gives a cross linked, infusible solid used in electrical switches.

Answer: (c) a, b and d

Explanation:
1. Fibres have the strongest intermolecular forces (hydrogen bonding or dipole attractions) and therefore high tensile strength.
2. Nylon, Dacron and polyesters are fibres.
3. Rubber is an elastomer with weak forces. So the answer is a, b and d.

Teacher's Note:
1. Order of forces: fibres > thermosetting plastics > thermoplastics > elastomers.
2. Dacron is itself a polyester, so (b) and (d) are the same family.

Answer: (b) i-b, ii-c, iii-a, iv-e, v-d

Explanation:
1. Glycol with phthalic acid gives Glyptal: i-b.
2. Butadiene with styrene gives Buna-S: ii-c.
3. Phenol with formaldehyde gives Novolac (and Bakelite): iii-a.
4. Glycol with terephthalic acid gives Dacron (Terylene): iv-e.
5. Butadiene with acrylonitrile gives Buna-N: v-d.

Teacher's Note:
1. Phthalic acid gives Glyptal and terephthalic acid gives Dacron; do not swap them.
2. S stands for styrene and N for nitrile.

Answer: (b) i-b, ii-a, iii-d, iv-c

Explanation:
1. Terylene is a polyester, so it has ester links: i-b.
2. Cellulose is a polymer of glucose joined by glycosidic links: ii-a.
3. Proteins have peptide (amide) links: iii-d.
4. RNA nucleotides are joined by phosphodiester links: iv-c.

Teacher's Note:
1. A peptide bond is simply an amide linkage.
2. Nucleic acids use phosphodiester links between sugars.

Answer: (b) \( \ce{AlCl3} \)

Explanation:
1. Cationic polymerisation needs an electrophilic initiator, such as a Lewis acid or a strong protic acid, which can add a positive centre to the monomer.
2. \( \ce{AlCl3} \) is a strong Lewis acid and is used with a trace of water. BuLi and \( \ce{LiAlH4} \) are nucleophiles, and \( \ce{HNO3} \) is an oxidising acid that would react with the monomer.

Teacher's Note:
1. Cationic initiators are Lewis acids; anionic initiators are strong nucleophiles such as BuLi.
2. Monomers with electron donating groups polymerise by the cationic route.

Answer: (b) \( \left[-\ce{CH2-C(Cl)=CH-CH2}-\right]_n \)

Explanation:
1. Neoprene is the polymer of chloroprene (2-chlorobuta-1,3-diene).
2. Addition of the diene units in a 1,4 manner leaves the double bond between the middle carbons, giving the repeating unit \( -\ce{CH2-C(Cl)=CH-CH2}- \).
3. Option 1 is polystyrene, option 3 is polyacrylonitrile and option 4 is natural rubber (polyisoprene).

Teacher's Note:
1. Neoprene is natural rubber with the methyl group replaced by chlorine.
2. It resists oil and heat better than natural rubber.

Answer: (c) Nylon-6 : Adipic acid

Explanation:
1. Natural rubber is a polymer of isoprene, so option 1 is correct.
2. Nylons contain amide links and are polyamides, so option 2 is correct.
3. Nylon-6 is made from caprolactam (or 6-aminohexanoic acid). Adipic acid is used for Nylon-6,6, so option 3 is wrong.
4. The Ziegler-Natta catalyst is titanium tetrachloride with triethylaluminium, so option 4 is correct.

Teacher's Note:
1. Nylon-6,6 uses hexamethylenediamine and adipic acid; Nylon-6 uses caprolactam.
2. The two numbers in a nylon name count the carbons of the diamine and the diacid.

Answer: (d) Antihistamine : Ranitidine

Explanation:
1. Nylon-6 is obtained from caprolactam, Buna-N is a synthetic rubber (elastomer) and alitame is an artificial sweetener, so the first three are correct.
2. Ranitidine is an antacid that blocks the histamine H2 receptors of the stomach. It is not an antihistamine of the allergy type (for example brompheniramine or terfenadine), so option 4 is the incorrect match.

Teacher's Note:
1. Allergy antihistamines block H1 receptors; antacids such as ranitidine block H2 receptors to reduce acid.
2. Do not confuse the two groups.

Answer: (c) Bakelite : thermoplastic

Explanation:
1. Nylon-6 has amide links and is a polyamide. Buna-S is made from two monomers and is a copolymer. Polythene is formed by addition of ethene units.
2. Bakelite is cross linked and cannot be remoulded, so it is thermosetting, not thermoplastic. Option 3 is the wrong match.

Teacher's Note:
1. Thermoplastics are linear or slightly branched; thermosetting plastics are cross linked.
2. Polythene, PVC and polystyrene are thermoplastics.

Answer: (a) cis-1,4-polyisoprene

Explanation:
1. Natural rubber is a linear polymer of isoprene (2-methylbuta-1,3-diene).
2. All the double bonds in the chain have the cis arrangement, so it is cis-1,4-polyisoprene.
3. The trans form is gutta-percha. Neoprene is made from chloroprene, not isoprene.

Teacher's Note:
1. Cis-1,4-polyisoprene is elastic because the cis chains are coiled.
2. The trans isomer is hard and non-elastic.

Answer: (b) 1,3-butadiene and acrylonitrile

Explanation:
1. Buna stands for butadiene and Na (sodium, the old catalyst); the letter N denotes acrylonitrile.
2. So Buna-N is made from 1,3-butadiene and acrylonitrile.

Teacher's Note:
1. Buna-S uses styrene in place of acrylonitrile.
2. Buna-N is resistant to oils and is used for oil seals and tank linings.

Answer: (a) 3-hydroxybutanoic acid and 3-hydroxypentanoic acid

Explanation:
1. PHBV is poly(3-hydroxybutyrate-co-3-hydroxyvalerate).
2. It is formed from 3-hydroxybutanoic acid and 3-hydroxypentanoic acid (3-hydroxyvaleric acid).
3. Both monomers are 3-hydroxy acids, which rules out the other options.

Teacher's Note:
1. PHBV is a biodegradable polyester used in packaging and in controlled drug release.
2. Valeric acid is the old name of pentanoic acid.

Answer: (b) Nylon-6,6

Explanation:
1. Nylon-6,6 forms when hexamethylenediamine reacts with adipic acid and water is eliminated, which is condensation polymerisation.
2. Polystyrene and Teflon are formed by addition of alkene monomers, and natural rubber is an addition polymer of isoprene.

Teacher's Note:
1. Condensation polymers lose a small molecule such as water at each step.
2. Nylons, polyesters and Bakelite are the usual examples.

Answer: (a) a linear condensation product of phenol and formaldehyde

Explanation:
1. With excess phenol and an acid catalyst, phenol and formaldehyde condense to give a linear polymer, Novolac.
2. Heating Novolac with more formaldehyde gives the cross linked Bakelite.

Teacher's Note:
1. Novolac is linear and fusible; Bakelite is the cross linked final product.
2. Urea formaldehyde resin is a different polymer.