140. Which statement about low-density polythene is false?
[JEE Main Offline Chemistry — 2016]
(A) It can be manufactured under high pressure
(B) It is usually an electrical insulator
(C) Peroxide or oxygen initiators may be used in its synthesis
(D) It is chiefly the material of rigid buckets and dustbins
Answer: (D) It is chiefly the material of rigid buckets and dustbins
Explanation: Low-density polyethylene is highly branched and is commonly used for flexible films and bags. High-density polyethylene is better suited to rigid articles such as buckets and dustbins. LDPE can be made by high-pressure radical polymerisation and normally has poor electrical conductivity.
139. Which polymer can be prepared by free-radical addition polymerisation?
[JEE Main Online 10 Apr Morning Chemistry — 2016]
(A) Teflon
(B) Terylene
(C) Melamine polymer
(D) Nylon-6,6
Answer: (A) Teflon
Explanation: Teflon, or polytetrafluoroethylene, is an addition polymer obtainable through chain-growth reactions of tetrafluoroethylene. Terylene and nylon-6,6 are condensation polymers, while melamine-formaldehyde resin also forms through a different network-forming process. Monomer structure determines suitable polymerisation mechanisms.
138. Which element forms the fundamental basis of all organic compounds?
[BPSC Prelims — 56th to 59th Combined Examination (2015)]
(A) Nitrogen
(B) Oxygen
(C) Carbon
(D) Brimstone
Answer: (C) Carbon
Explanation: Organic compounds are broadly defined by carbon-containing covalent molecular structures, frequently with hydrogen and also sometimes oxygen, nitrogen or other elements. Carbon can form four bonds and link repeatedly with itself. This combination supports the large diversity of biological and synthetic organic molecules.
137. Which element is most fundamental to the structure of all bio-compounds?
[UPPCS Mains — 2015]
(A) Sulphur
(B) Carbon
(C) Oxygen
(D) Nitrogen
Answer: (B) Carbon
Explanation: Carbon forms the core skeleton of carbohydrates, lipids, proteins and nucleic acids, permitting numerous stable covalent structures. Oxygen and nitrogen are common in biomolecules, but not all compounds contain each of them. Carbon is the foundational element of organic biochemistry.
136. Coal, petroleum and natural gas are collectively examples of:
[WBCS Prelims — 2015]
(A) Fossil fuels
(B) Cryogenic fuels
(C) Indigenous fuels
(D) Radioactive fuels
Answer: (A) Fossil fuels
Explanation: Coal, crude petroleum and natural gas are fossil fuels formed from ancient organic matter under geological processes over millions of years. Natural gas is rich in methane; petroleum contains a mixture of hydrocarbons; and coal consists largely of carbon-rich material.
135. Which substituent is meta-directing in electrophilic aromatic substitution?
[RPSC Senior Teacher Science — 2015]
(A) –NH₂
(B) –SO₃H
(C) –OCH₃
(D) –C₂H₅
Answer: (B) –SO₃H
Explanation: The sulfonic acid group withdraws electron density from the benzene ring, destabilising ortho and para substitution pathways relative to meta attack. Hence –SO₃H is meta-directing. Amino, methoxy and alkyl groups generally favour ortho and para electrophilic aromatic substitution.
134. Neoprene synthetic rubber is polymerised from which monomer?
[RPSC Senior Teacher Science — 2015]
(A) Isoprene
(B) Chloroprene
(C) Caprolactam
(D) Acrylonitrile
Answer: (B) Chloroprene
Explanation: Neoprene, or polychloroprene, results from the polymerisation of chloroprene. It is an artificial elastomer noted for useful resistance to oil, weathering and many chemicals. Isoprene is associated with natural rubber, while caprolactam is used for nylon-6.
133. Which amino acid cannot be synthesised sufficiently by the human body and must be obtained from food?
[RPSC Senior Teacher Science — 2015]
(A) Glycine
(B) Alanine
(C) Tryptophan
(D) Proline
Answer: (C) Tryptophan
Explanation: Tryptophan is an essential amino acid needed from the diet because humans cannot synthesise it adequately. Glycine, alanine and proline can ordinarily be synthesised in the body. Amino acids contain both amino and carboxyl functional groups and form the building blocks of proteins.
132. What stereochemical result is most commonly observed for SN1 substitution at a chiral centre?
[AIPMT Chemistry — 2015]
(A) More inversion than retention, with partial racemisation
(B) Complete retention only
(C) Complete inversion only
(D) Always precisely 50:50 racemisation
Answer: (A) More inversion than retention, with partial racemisation
Explanation: The SN1 mechanism forms a largely planar carbocation that can be attacked from either face. Solvent cages or ion-pair effects may shield one face, so a slight excess of inversion often occurs. The product can therefore show partial racemisation instead of perfect 50:50 mixing.
131. Which polymer is used in formulating paints and lacquers?
[JEE Main Offline Chemistry — 2015]
(A) Glyptal
(B) Polypropene
(C) Polyvinyl chloride
(D) Bakelite
Answer: (A) Glyptal
Explanation: Glyptal is an alkyd polyester resin produced from polyhydric alcohols and polybasic acids or anhydrides. Such resins are used as binding materials in protective coatings, paints and lacquers. Its film-forming characteristics distinguish it from ordinary packaging plastics.
