Polymers and Organic Chemistry

An organic structure is a map of connections. A functional group is the atom or group of atoms responsible for a characteristic set of reactions. First find the functional group; then identify the carbon skeleton. The same molecular formula can describe compounds with different connections and therefore different chemistry.

To turn a condensed formula into a displayed formula, draw the carbon skeleton first, then expand every bracketed group at its stated attachment point. Replace COOH by C(=O)-O-H and CHO by C(=O)-H. Finally add each C-H bond explicitly and check that each ordinary neutral carbon has four bonds, each oxygen two, and each hydrogen one. In a skeletal formula, apply the same valency check to recover the hidden C-H bonds.

Here R and R' represent carbon-containing groups; they need not be identical. In methanal and methanoic acid, the group attached to the carbonyl carbon can be H instead. General molecular formulae apply only with the stated restrictions: rings, extra double bonds or extra functional groups change them. A formula alone does not establish the family; for example, propanal and propanone both have C₃H₆O.

For straightforward names, choose a parent chain containing the principal functional group and number it to locate that group or the double bond with a low number. The prefixes meth-, eth-, prop-, but-, pent- and hex- indicate 1-6 carbons. Show branches and substituent positions: CH₃CH(CH₃)CH₂OH is 2-methylpropan-1-ol. Count the carbon attached to OH as carbon 1; the methyl branch is then on carbon 2.

Constitutional (structural) isomers have the same molecular formula but different atom-to-atom connections. You can change the carbon skeleton, move a functional group to an inequivalent position, or use a different functional group compatible with the formula. Rotating or reversing the same drawn chain does not make a new constitutional isomer.

Cis-trans isomerism keeps the same connectivity but changes the spatial arrangement around a C=C bond. A double bond contains a sigma bond and a pi bond. Free rotation would destroy the sideways orbital overlap forming the pi bond, so the arrangement is restricted. Each double-bonded carbon must have two different substituents for cis-trans alternatives to exist.

A multiple bond counts as one direction of electron density when predicting local shape. Ethene has a double bond but only three directions around each carbon, so it is trigonal planar. No hybridisation labels are needed here: use the number and arrangement of sigma bonds and explain where sideways pi overlap is possible.

Alkanes have only strong C-C and C-H sigma bonds and no especially reactive functional group. They are generally unreactive under ordinary conditions, but burn and undergo substitution with chlorine under ultraviolet light. Alkenes contain a pi bond, which can be replaced by two new sigma bonds in an addition reaction. Addition joins reactants into a product without eliminating a small molecule; substitution replaces an atom or group with another.

Elimination removes atoms or groups from adjacent positions to form a multiple bond. In contrast, substituting Br with OH keeps the carbon-carbon skeleton saturated. For bromoethane, the solvent and conditions make the requested product clear; writing only "NaOH" leaves the answer incomplete.

A reflux condenser returns vaporised volatile material to the reaction vessel, permitting sustained heating without continually losing that material. When oxidising ethanol to the acid, sufficient oxidant and continued contact allow oxidation to proceed through the aldehyde stage to the carboxylic acid. Acidified dichromate changes from orange towards green; acidified permanganate loses its purple colour as it is reduced under the stated conditions.

Organic oxidation often increases the number of bonds from carbon to oxygen and/or decreases the number of C-H bonds. Reduction does the reverse. [O] and [H] represent oxygen or hydrogen equivalents in an organic redox equation; they are bookkeeping symbols, not bottles of isolated atoms.

In carbonyl reduction, one hydrogen is added to the carbonyl carbon and one to oxygen, converting C=O into CH-OH. An aldehyde therefore gives a primary alcohol; a ketone gives a secondary alcohol. LiAlH₄ is used in dry conditions, commonly dry ether, before the subsequent aqueous work-up; writing it as an aqueous reagent at the reduction stage is inappropriate. An aldehyde can be oxidised to a carboxylic acid without breaking its carbon skeleton. A ketone resists these usual aldehyde-oxidation conditions; do not invent a same-skeleton carboxylic acid product from it.

Ethanoic acid donates a proton from COOH. With an alkali it forms a carboxylate salt and water; with a carbonate it forms salt, water and carbon dioxide. The group changes from -COOH to -COO^-; the carbon skeleton is retained.

Condensation joins molecules with elimination of a small molecule, often water. Hydrolysis uses water to break a bond; aqueous acid or alkali and heat can facilitate it. Ester and amide links connect carbonyl-containing fragments to O or N respectively. Find that connecting bond before trying to name the products.

The ester link -C(=O)-O- is neutral: the final dash is a covalent bond continuing to a carbon group. In a carboxylate ion, -COO^-, the superscript minus is a charge. These are different structures. In particular, ester links in a polyester backbone do not give the unchanged polymer a negative charge.

Product isolation is a reasoning task rather than a memorised apparatus list. After making an ester, ask which reagents remain: excess acid, acid catalyst, alcohol and water can all accompany the product. An aqueous wash can remove water-soluble impurities; an appropriate carbonate wash can neutralise acid. If two liquid layers form, separation can isolate the ester-containing layer; drying then removes traces of water. Use supplied solubility and boiling-point data to choose further separation, and never identify a layer solely from whether it is on top.

A polymer is a macromolecule built from many monomer units. For this course, recognise a polymer as having an average relative molecular mass of at least 1000 or at least 100 repeat units. A polymer sample normally contains chains of different lengths, so an average is useful; the subscript n does not mean every chain has exactly the same length.

For poly(ethene), nCH₂=CH₂ → [-CH₂-CH₂-]_n. The bonds at both sides of the repeat unit must continue through the brackets. To infer an alkene monomer from a simple addition repeat unit, isolate two adjacent backbone carbons, restore their double bond and keep their substituents attached. Do not break a C-H bond to make the monomer.

An alpha-amino acid has NH₂ and COOH attached to the same carbon: H₂N-CH(R)-COOH. Aminoethanoic acid has R = H. In a protein, the carboxyl group of one amino acid joins the amino group of another, giving a peptide bond, -C(=O)-NH-. A peptide bond is an amide link; repeated condensation builds a chain with a repeating -NH-CH(R)-CO- backbone.

The amino-acid sequence provides a covalently bonded backbone, but many functions depend on the chain adopting a particular three-dimensional shape. Interactions between different parts of the chain, or between different chains, help stabilise that shape. An enzyme's active site depends on this arrangement, linking protein structure to the specificity explained in Reaction Kinetics.

Denaturation is loss of the specific three-dimensional structure responsible for normal function. High temperature can disrupt the interactions maintaining that arrangement; extreme pH changes protonation and therefore ionic attractions and other interactions. A protein can unfold, refold incorrectly or aggregate with other chains. The usual denaturation explanation does not require cleavage of the peptide backbone.

Low temperature usually slows enzyme-catalysed reactions by reducing molecular motion; it does not automatically denature the enzyme. At excessively high temperature the enzyme may lose its active-site shape, so fewer enzyme-substrate complexes form even though molecules move faster. Returning to the original temperature or pH does not guarantee recovery after aggregation or other irreversible changes.

Protein hydrolysis is a different chemical change: heat with aqueous acid or aqueous alkali breaks peptide links, ultimately giving amino-acid-derived products after complete hydrolysis. Acidic conditions give protonated amino groups, H₃N⁺-CH(R)-COOH. Alkaline conditions give carboxylate groups, H₂N-CH(R)-COO^-, with the appropriate counter-ions. The amino-acid skeletons and side chains are recovered, subject to the supplied conditions.

A polymer contains strong covalent bonds along its backbone. Between separate chains are weaker intermolecular attractions. A thermoplastic, exemplified by linear poly(ethene), softens when enough interchain attractions are overcome to allow chains to move past one another. Cooling can restore a solid shape. This remoulding normally occurs without breaking the backbone into monomers.

A thermosetting polymer, exemplified by cross-linked poly(diallyl phthalate), has covalent links between chains, forming a network. The crosslinks restrict chain movement. After curing, heating cannot simply separate the chains into a flowing melt; sufficiently severe heating instead breaks or decomposes the network. It cannot be recycled by straightforward remelting in the way a suitable thermoplastic can.

LDPE and HDPE are both poly(ethene), but their chain arrangements differ. LDPE has more branching, which makes close regular packing harder. Fewer close interchain contacts per volume give lower density and a softer, more flexible material, useful for plastic bags. HDPE has less branching and more closely packed regions. More effective interchain attractions give greater stiffness and hardness, useful for plastic bottles. The difference is not that HDPE has stronger C-C covalent bonds.

A useful material comparison needs a chain of reasoning: identify the structural difference, explain the different interactions or reactions, then connect that difference to the required use. "It is strong" or "it is polar" alone does not explain why one named polymer is the better choice.

PET fabric is slightly less prone to creasing than nylon 6,6 in the syllabus comparison. Nylon amide groups can form many interchain hydrogen bonds. During deformation and exposure to moisture, some original contacts can be disrupted and new contacts formed in displaced positions, helping retain a crease. PET has polar ester groups but no repeated N-H or O-H hydrogen-bond donors along the ideal backbone, so this particular interchain hydrogen-bonding effect is smaller. PET still has intermolecular attractions; neither polymer is completely crease-proof, and fabric construction and treatment also matter.

PVA can be water-soluble and is used in eye drops. Its many OH groups form favourable hydrogen bonds with water, allowing suitable chains to become hydrated and dispersed. PVC is water-resistant and is used in raincoats. Its C-Cl bonds are polar, but polarity by itself does not ensure dissolution: it lacks PVA's extensive OH-water hydrogen bonding, and interactions with water do not sufficiently compensate for separating the chains. Covalently bound chlorine here is not equivalent to a hydrated chloride ion.

Choose PP rather than PET for strongly alkaline cleaning solutions. The alkali can hydrolyse PET's ester links, cutting its backbone into shorter fragments and weakening the container. PP has no hydrolysable ester links and its hydrocarbon chain is relatively resistant to this chemical attack. This is a chemical-resistance decision; stiffness alone does not determine suitability.

Poly(alkenes) have robust C-C backbones and mostly C-H bonds. Their relative chemical inertness is useful for containers and waterproof materials, but it makes many of them difficult to biodegrade: ordinary aqueous conditions and biological processes do not readily cleave those backbones into small usable molecules. Fragmenting into smaller pieces is not the same as complete biodegradation.

Polyesters and polyamides contain hydrolysable ester or amide links, so they are generally more susceptible to biodegradation through hydrolysis than comparable poly(alkenes). Cutting these links reduces chain length and can produce smaller molecules that further biological processes may use. A hydrolysable link does not guarantee rapid breakdown in every environment. Temperature, water access, crystallinity, chain structure and suitable biological activity affect the rate. A durable PET bottle and a readily degradable polyester can both contain ester links.

Materials and the resources used to produce them are finite. Recycling can reduce demand for fresh feedstock and reduce waste, but it should be evaluated across the whole process. A persuasive comparison follows the material and energy through collection, sorting, transport, cleaning, processing and its next useful life, rather than stopping at the recycling symbol.

Named materials: LDPE branches pack less closely than HDPE; nylon 6,6 has stronger interchain hydrogen-bonding opportunities than PET; PVA OH groups hydrate with water whereas PVC is water-resistant; PP resists the ester hydrolysis that can damage PET in strong alkali.

Polyalkene inertness makes biodegradation difficult. Ester/amide links permit hydrolysis, but real rates depend on conditions. Recycling decisions must include economic, environmental and social consequences, not only a recovery percentage.