Polymers and Organic Chemistry

An organic structure is a map of which atoms are joined. A functional group is the atom or group of atoms that gives a compound its typical reactions. First find the functional group, then the carbon chain. The same molecular formula can describe compounds with different connections, and so different chemistry.

To turn a condensed formula into a displayed formula, draw the carbon chain first. Then draw out each bracketed group where it is attached. Write COOH as C(=O)-O-H and CHO as C(=O)-H. Finally draw every C-H bond and check the bonds: four for each carbon, two for each oxygen and one for each hydrogen. In a skeletal formula, use the same check to find the hidden C-H bonds.

Here R and R' stand for groups that contain carbon. They do not have to be the same. In methanal and methanoic acid, the group on the carbonyl carbon is H instead. General formulae only work under their stated conditions. Rings, extra double bonds or extra functional groups change them. A formula alone does not tell you the family: propanal and propanone are both C₃H₆O.

To name a simple compound, choose the parent chain that contains the main functional group. Number it so that the group or the double bond gets a low number. The stems meth-, eth-, prop-, but-, pent- and hex- mean 1 to 6 carbons. Show the branches and their positions. CH₃CH(CH₃)CH₂OH is 2-methylpropan-1-ol: the carbon joined to OH is carbon 1, so the methyl branch is on carbon 2.

Constitutional (structural) isomers have the same molecular formula, but their atoms are joined differently. You can change the carbon chain, move the functional group to a different position, or use a different functional group that fits the formula. Turning a chain round or drawing it the other way does not make a new isomer.

Cis-trans isomerism keeps the same connections but changes the arrangement in space around a C=C bond. A double bond is one sigma bond and one pi bond. Rotating it would break the sideways overlap that forms the pi bond, so it cannot rotate freely. Each carbon of the double bond must carry two different groups for cis and trans forms to exist.

A multiple bond counts as one direction of electron density when you predict the shape around an atom. In ethene, each carbon has a double bond but only three directions, so it is trigonal planar. You do not need hybridisation labels here. Use the number and arrangement of sigma bonds, and say where sideways pi overlap can happen.

Alkanes have only strong C-C and C-H sigma bonds and no reactive functional group. So they are unreactive under ordinary conditions. They do burn, and they react with chlorine by substitution in ultraviolet light. Alkenes have a pi bond, which can be replaced by two new sigma bonds in an addition reaction. Addition joins reactants into one product and loses no small molecule. Substitution replaces one atom or group with another.

Elimination removes atoms or groups from two neighbouring carbons and forms a double bond. Substituting Br with OH is different: the carbon chain stays saturated. For bromoethane, the solvent and conditions decide the product, so writing only "NaOH" is not a complete answer.

A reflux condenser cools the vapour so that it drips back into the flask. This lets you heat for a long time without losing volatile substances. When ethanol is oxidised to the acid, enough oxidant and a long heating time let the reaction go past the aldehyde to the carboxylic acid. Acidified dichromate turns from orange to green. Acidified permanganate loses its purple colour as it is reduced.

In organic chemistry, oxidation usually adds bonds from carbon to oxygen or removes C-H bonds. Reduction does the opposite. [O] and [H] stand for the oxygen or hydrogen supplied in a balanced equation. They are a way of keeping count, not real single atoms in a bottle.

In carbonyl reduction, one hydrogen adds to the carbonyl carbon and one to the oxygen, so C=O becomes CH-OH. An aldehyde gives a primary alcohol, and a ketone gives a secondary alcohol. LiAlH₄ is used in dry conditions, usually dry ether, and water is added afterwards. Do not write it as an aqueous reagent. An aldehyde can be oxidised to a carboxylic acid with the same carbon chain. A ketone is not oxidised by these usual conditions, so do not give it a carboxylic acid product.

Ethanoic acid gives a proton from its COOH group. With an alkali it forms a carboxylate salt and water. With a carbonate it forms a salt, water and carbon dioxide. The group changes from -COOH to -COO^-, and the carbon chain stays the same.

Condensation joins molecules and removes a small molecule, often water. Hydrolysis uses water to break a bond. Heating with aqueous acid or alkali speeds it up. Ester links join a carbonyl group to O, and amide links join it to N. Find that linking bond before you try to name the products.

The ester link -C(=O)-O- is neutral. The dash at the end is a covalent bond to the next carbon group. In a carboxylate ion, -COO^-, the minus sign is a charge. These are different structures. So the ester links in a polyester do not give the polymer a negative charge.

Separating the product is about reasoning, not a memorised list of apparatus. After making an ester, ask what else is left: excess acid, acid catalyst, alcohol and water may all be present. Washing with water removes impurities that dissolve in water. Washing with a carbonate solution neutralises acid. If two liquid layers form, separate the layer that holds the ester, then dry it to remove the last water. Use the solubility and boiling-point data given to choose further steps. Never decide which layer is which only from whether it is on top.

A polymer is a very large molecule built from many monomer units. In this course, a molecule counts as a polymer if its average relative molecular mass is at least 1000 or it has at least 100 repeat units. A polymer sample contains chains of different lengths, so an average is used. The n in the formula does not mean every chain has exactly the same length.

For poly(ethene), nCH₂=CH₂ → [-CH₂-CH₂-]_n. The bonds on both sides of the repeat unit must go through the brackets. To find the monomer from an addition repeat unit, take two neighbouring backbone carbons, put back their double bond and keep their groups attached. Do not break a C-H bond to make the monomer.

An alpha-amino acid has NH₂ and COOH on the same carbon: H₂N-CH(R)-COOH. In aminoethanoic acid, R = H. In a protein, the carboxyl group of one amino acid joins the amino group of the next, forming a peptide bond, -C(=O)-NH-. A peptide bond is an amide link. Repeating this condensation builds a chain with a repeating -NH-CH(R)-CO- backbone.

The order of amino acids gives a chain held together by covalent bonds. But many proteins only work when the chain folds into a particular three-dimensional shape. Attractions between different parts of the chain, or between chains, hold that shape. An enzyme's active site depends on this shape, which links protein structure to the specificity explained in Reaction Kinetics.

Denaturation is the loss of the particular three-dimensional shape that a protein needs to work. High temperature can break the attractions that hold this shape. Extreme pH changes which groups carry a charge, so ionic attractions and other interactions change. The protein can unfold, refold wrongly or clump with other chains. Denaturation does not break the peptide bonds of the backbone.

Low temperature usually slows enzyme reactions because molecules move more slowly. It does not denature the enzyme. At too high a temperature, the enzyme can lose the shape of its active site. Then fewer enzyme-substrate complexes form, even though molecules move faster. Going back to the original temperature or pH does not always bring the activity back, for example if the protein has clumped.

Protein hydrolysis is a different chemical change. Heating with aqueous acid or aqueous alkali breaks the peptide links, and complete hydrolysis gives amino acids. In acid, the amino groups are protonated: H₃N⁺-CH(R)-COOH. In alkali, the acid groups become carboxylate ions: H₂N-CH(R)-COO^-, with suitable counter-ions. The carbon chains and side groups of the amino acids are recovered.

A polymer has strong covalent bonds along its backbone. Between separate chains there are weaker intermolecular attractions. A thermoplastic, such as straight-chain poly(ethene), softens when heating overcomes enough of these attractions for the chains to slide past each other. Cooling makes it solid again. This reshaping does not break the backbone into monomers.

A thermosetting polymer, such as cross-linked poly(diallyl phthalate), has covalent bonds between its chains, which form a network. These cross-links stop the chains moving. Once it has set, heating cannot separate the chains into a liquid. Strong heating breaks down the network instead. So it cannot be recycled by simply melting it again, as a suitable thermoplastic can.

LDPE and HDPE are both poly(ethene), but their chains are arranged differently. LDPE has more branches, which stop the chains packing closely. Fewer close contacts between chains give a lower density and a softer, more flexible material, useful for plastic bags. HDPE has fewer branches, so its chains pack more closely. Stronger attractions between chains make it stiffer and harder, useful for plastic bottles. HDPE does not have stronger C-C bonds.

A good comparison of materials is a chain of reasoning. Name the difference in structure. Explain how that changes the interactions or reactions. Then link this to the use. Writing only "it is strong" or "it is polar" does not explain why one polymer is the better choice.

PET fabric creases slightly less than nylon 6,6 in the syllabus comparison. Nylon has amide groups that form many hydrogen bonds between chains. When the fabric is bent, especially when damp, some of these bonds break and new ones form in the new position, which holds the crease in. PET has polar ester groups but no N-H or O-H groups along its chain to donate hydrogen bonds, so this effect is smaller. PET still has attractions between chains. Neither fabric is fully crease-proof, and how the fabric is made and treated also matters.

PVA can dissolve in water and is used in eye drops. Its many OH groups form hydrogen bonds with water, so the chains can become surrounded by water and spread out. PVC is water-resistant and is used in raincoats. Its C-Cl bonds are polar, but being polar is not enough to dissolve. It does not have PVA's many OH groups to hydrogen bond with water, so water cannot pull its chains apart. The chlorine is covalently bonded, so it is not like a chloride ion in solution.

Choose PP rather than PET for strongly alkaline cleaning solutions. Alkali can hydrolyse the ester links in PET, cutting its chains into shorter pieces and weakening the container. PP has no ester links to hydrolyse, and its hydrocarbon chain resists this attack. This choice is about chemical resistance. Stiffness alone does not decide it.

Poly(alkenes) have strong C-C backbones with mostly C-H bonds. Being so unreactive makes them useful for containers and waterproof materials. But it also makes many of them hard to biodegrade, because water and living things cannot easily cut those chains into small molecules. Breaking into small pieces is not the same as biodegrading completely.

Polyesters and polyamides have ester or amide links that can be hydrolysed. So they usually biodegrade more easily than poly(alkenes). Cutting these links shortens the chains and can give small molecules that living things can use. A link that can be hydrolysed does not guarantee fast breakdown everywhere. Temperature, water, how crystalline the polymer is, the chain structure and suitable microorganisms all affect the rate. A long-lasting PET bottle and a fast-degrading polyester both contain ester links.

Materials and the resources used to make them are limited. Recycling can reduce the need for new raw materials and cut waste, but judge the whole process. A good comparison follows the material and the energy through collection, sorting, transport, cleaning, processing and its next use. Do not stop at the recycling symbol.

Named materials: LDPE branches pack less closely than HDPE. Nylon 6,6 can form more hydrogen bonds between chains than PET. PVA OH groups bond with water, while PVC resists water. PP resists the ester hydrolysis that can damage PET in strong alkali.

Poly(alkenes) are unreactive, so they biodegrade with difficulty. Ester and amide links can be hydrolysed, but real rates depend on conditions. Recycling decisions must weigh economic, environmental and social effects, not only how much is recovered.