BPSC 71st CCE Prelims General Studies · Question 138 of 150

Correct — C, Poly (3 - Hydroxy butyrate-co-3 hydroxy valerate). The acronym decodes letter by letter: P for poly, HB for 3-hydroxybutyrate, V for 3-hydroxyvalerate. PHBV belongs to the polyhydroxyalkanoates or PHAs — a family of polyesters that are not manufactured from petrochemicals at all but produced inside living cells, by bacterial fermentation of sugars or lipids, and stored as intracellular granules that serve the microbe as both an energy source and a carbon store. More than 150 different monomers can be combined within the family, giving materials of very different character with melting points from 40 to 180 °C, and the polymer harvest from those granules can reach 80% of the organism's dry weight. Two details of option (c) prove it is the real chemical name rather than a plausible string. First, the locant '3-' in front of each monomer states exactly where the hydroxyl group sits — on the third carbon of a four-carbon butanoate unit and of a five-carbon pentanoate (valerate) unit. Second, the infix 'co' is IUPAC copolymer notation: poly(A-co-B) means a single chain built from both repeating units, not a blend of two separate polymers. That copolymer structure is the entire reason PHBV exists commercially. Pure poly-3-hydroxybutyrate, PHB, was the first PHA discovered and it is a disappointing plastic — highly crystalline, stiff and brittle, melting at 175 °C with a glass transition of only 2 °C and a tensile strength near 40 MPa, close to polypropylene's, but far too brittle to process comfortably. Threading hydroxyvalerate units into the chain disrupts the crystal packing, and crystallinity across the PHA family ranges from a few per cent to about 70%: PHBV is accordingly less stiff and tougher than PHB, and its processability, impact strength and flexibility all improve as the valerate fraction rises. That toughening is why PHBV, and not PHB, is used as a packaging material — and why the examiner chose it as the model biodegradable polymer.

• (a)Polyhydroxy butyl vaniline — Vanillin — spelt 'vaniline' here — is a genuine compound, 4-hydroxy-3-methoxybenzaldehyde, the molecule that gives vanilla its flavour, and its aromatic ring does appear in some experimental bio-polymers. But it is an aldehyde, not a hydroxy acid, so it cannot form the ester linkages that define a polyhydroxyalkanoate. 'Butyl' is also wrong in kind: an alkyl substituent, not the butyrate ester unit the B stands for.
• (b)Poly hydroxy butane veratric acid — Two independent errors, either of which is fatal. Butane is a saturated alkane with no functional group at all, so nothing about it can polymerise into a polyester — the monomer is butyrate, the butanoate anion. And veratric acid, 3,4-dimethoxybenzoic acid, is an aromatic plant acid that has nothing to do with the aliphatic five-carbon valerate the V denotes.
• (d)Poly hydroxy butyric acid veratric acid — The near-miss, and the option that catches a candidate with partial knowledge. Its first half is genuinely right — 3-hydroxybutyric acid is the HB monomer. It then substitutes veratric acid for valeric acid, and the V in PHBV is valerate (pentanoate, five carbons), not veratric. It also omits the 'co' infix, so even read charitably the name describes two acids rather than one copolymer chain.

Most plastics in use are synthetic polymers made from petrochemicals, and they resist biodegradation because no common enzyme recognises their carbon backbones. Polyhydroxyalkanoates take the opposite route: they are made by micro-organisms, they are true polyesters, and soil and sludge bacteria of the genera Bacillus, Pseudomonas and Streptomyces can eat them back down. Production is a fermentation, not a cracking plant. A culture such as Cupriavidus necator is grown to high density on glucose, sucrose, vegetable oil or biodiesel glycerine, and is then deliberately starved of a nutrient — nitrogen, phosphorus, a trace element or oxygen — while carbon is kept in excess. The stressed cells respond by laying down PHA granules, which are recovered by rupturing the cells. Keep two distinctions straight, because examiners exploit both. Bio-BASED is not the same as biodegradable: bio-PET made from sugarcane ethanol is renewable in origin yet persists exactly like fossil PET, while PBAT is petroleum-derived and still compostable. And biodegradable is not the same as compostable, which is a certified performance claim about breaking down under defined industrial composting conditions in a defined time. PHAs happen to satisfy all three descriptions at once — bio-based, biodegradable and compostable — which is why they are the standard classroom example, and PHBV is the standard example within the family.

This is a pure decoding question, and it can be answered without ever having met PHBV, by reading the acronym against each option. P–H–B–V has four parts, so the correct expansion must supply four: poly, hydroxy, a B-word and a V-word, and both the B-word and the V-word must be things a polyester can actually be built from — that is, hydroxy acids. Option (a) offers vanillin for the V, an aromatic aldehyde. Options (b) and (d) both offer veratric acid, an aromatic benzoic acid. Only option (c) supplies valerate, an aliphatic five-carbon hydroxy acid that pairs naturally with the four-carbon butyrate. That single discrimination — V is for VALERATE — decides the mark. A second tell is stylistic and worth internalising for every 'full form' item in chemistry: real IUPAC names carry apparatus that invented names never bother to fake. Option (c) has numeric locants ('3-'), enclosing brackets, and the copolymer infix 'co'; the other three are strings of chemistry-sounding words with no positional information at all. When one option in a chemistry naming question is visibly more structured than the rest, that structure is usually not decoration. Finally, note what the question is not asking. It does not ask what PHBV is used for, how it is made or how fast it degrades — only what the letters stand for — so resist the urge to reason from properties.

• PHAs are polyesters produced in nature by micro-organisms through bacterial fermentation of sugars or lipids, serving the cell as both energy source and carbon store; more than 150 monomers can be combined within the family, melting points run from 40 to 180 °C, and the yield from intracellular granules can reach 80% of the organism's dry weight.
• Polyhydroxybutyrate was first isolated and characterised in 1925 by the French microbiologist Maurice Lemoigne. PHB melts at 175 °C, has a glass transition temperature of 2 °C and a tensile strength of about 40 MPa — close to polypropylene — but is brittle; unlike polypropylene it sinks in water, which aids its anaerobic breakdown in sediments.
• Imperial Chemical Industries developed poly(3-hydroxybutyrate-co-3-hydroxyvalerate) by fermentation in the 1980s and sold it as 'Biopol', distributed in the United States by Monsanto and later by Metabolix, which won the US Presidential Green Chemistry Challenge Award in June 2005 for a cost-effective PHA manufacturing route.
• Adding hydroxyvalerate units breaks up PHB's crystal packing — crystallinity across the PHA family ranges from a few per cent to about 70% — making PHBV less stiff and tougher, with processability, impact strength and flexibility all improving as the valerate percentage rises. PHAs are also UV-stable, unlike polylactic acid, and show low water permeation.
• Nomenclature to carry into the hall: in poly(A-co-B) the infix 'co' marks a copolymer built of both units in one chain; butyrate is the four-carbon butanoate unit and valerate the five-carbon pentanoate unit; and the '3-' locants place the hydroxyl group on the third carbon of each monomer.

• Reading the V as vanillin or veratric acid — it is valerate, the five-carbon (pentanoate) hydroxy acid, paired with four-carbon butyrate
• Treating PHBV as a blend of two polymers. The 'co' means one chain carrying both repeat units, and that is precisely what removes PHB's brittleness
• Assuming every bioplastic is biodegradable, or that every biodegradable plastic is bio-based — the two properties are independent, and exam statements routinely swap them

BPSC asks for the expansion itself — a flat 'full form of X' with one real IUPAC name hidden among invented look-alikes, so the mark turns on decoding letters rather than on understanding polymer science. UPSC has never asked a bare full form here. It names a polymer and then tests statements about its behaviour: what PET can and cannot safely hold and whether it is recyclable, which plastic bisphenol A actually builds, whether chewing-gum base counts as plastic. Learn the expansion for BPSC and the properties for UPSC.

• practice — not a real PYQ

  Polyhydroxyalkanoates (PHAs) such as PHBV are produced commercially by

  • (a)cracking of naphtha in petroleum refineries
  • (b)fermentation, in which bacteria store the polyester as intracellular granules
  • (c)polymerisation of ethylene at high pressure
  • (d)chemical modification of natural rubber latex

  Answer(b) fermentation, in which bacteria store the polyester as intracellular granules — cultures such as Cupriavidus necator are grown on sugars or vegetable oil, then starved of nitrogen or phosphorus while carbon stays in excess, so they lay down PHA granules that can reach 80% of the cell's dry weight.

• practice — not a real PYQ

  In the polymer name poly(3-hydroxybutyrate-co-3-hydroxyvalerate), the infix 'co' indicates that the material is a

  • (a)physical blend of two separate polymers
  • (b)copolymer whose single chain contains both repeat units
  • (c)polymer dissolved in a co-solvent
  • (d)cross-linked thermosetting resin

  Answer(b) copolymer whose single chain contains both repeat units — poly(A-co-B) is the standard notation for a copolymer, and it is the inclusion of hydroxyvalerate units in the chain that lowers crystallinity and makes PHBV tougher and less brittle than PHB.