Descriptive, Organic, and Nuclear Chemistry

This is Unit Twelve of Chemistry, Part Four: organic chemistry. The three parts before this read behaviour off the periodic table. This one narrows to a single element and finds that one element is enough to need a chapter of its own, because carbon does something no other element does at the same scale — it bonds to itself, repeatedly, in chains and branches and rings. Two ideas organise what follows. The first is that a molecule's reactivity is set by whether it has a multiple bond available to react across, which sorts organic reactions into two types that are easy to confuse and worth keeping apart. The second is that a small cluster of atoms attached to a carbon chain — a functional group — determines the compound's chemistry far more than the length of the chain does, which is why organic compounds are organised into families rather than listed. By the end you will be able to say what makes a compound organic and which carbon compounds are excluded; name an alkane by the IUPAC rules, including the tie-break; say why alkenes undergo addition where alkanes undergo substitution; tell an aldehyde from a ketone; and determine an organic carbon's oxidation level without a full oxidation-number calculation.

Four entries laying the ground. What carbon does that makes this subject possible, the simplest family of compounds that follows from it, the fact that one molecular formula can describe more than one compound, and the naming system built to resolve exactly that ambiguity.

Carbon bonding to carbon.

Chem 2429 Organic Compounds Defined by Carbon Catenation

An organic compound is built around carbon's ability to form up to four strong bonds, including to other carbons — catenation — producing chains, branches and rings. Carbon links into an alkane's long chains, benzene's six-membered ring, and everything between. Being carbon-containing is not sufficient, though: oxides like carbon dioxide and ionic compounds like carbonates are conventionally excluded.

Single bonds only.

Chem 2432 Alkanes: Saturated Hydrocarbons

An alkane is a saturated hydrocarbon with only single carbon–carbon bonds, each carbon sp3 hybridised, giving the general formula C-n-H-two-n-plus-two for any non-cyclic chain. Propane fits exactly: three carbons, eight hydrogens. Saturated is a statement about bonding and not about physical state — saturated hydrocarbons include gases, liquids and solids depending on chain length.

Same formula, connected differently.

Chem 2435 Structural Isomers Defined

Structural isomers, also called constitutional isomers, share a molecular formula and differ in how their atoms are connected. Butane and two-methylpropane are both four carbons and ten hydrogens, one unbranched and one branched. Two drawings that differ are not automatically two isomers — the same chain drawn straight, bent or coiled is one molecule, since nothing about the connectivity changed.

Longest chain, lowest numbers.

Chem 2438 IUPAC Alkane Naming

Find the longest continuous carbon chain for the base name, number it from the end nearest a substituent, and list substituents alphabetically with the position numbers kept as low as possible. A four-carbon chain with bromine on the second carbon and chlorine on the first is numbered from the chlorine end, giving two-bromo-one-chlorobutane. Where both directions give the identical set of numbers, the tie breaks alphabetically: the substituent cited first gets the lower number, so a seven-carbon chain tied at two and six is two-bromo-six-methylheptane. Numbering from whichever end was drawn first produces a wrong name even when every substituent is correctly identified.

Three entries on what happens when carbons share more than one pair of electrons. A double bond locks the two carbons against rotation, which creates a kind of isomerism that saturated chains cannot have; and a triple bond straightens that section of the molecule out entirely while making it more reactive rather than less.

Locked, not free.

Chem 2442 Alkenes: Restricted Rotation at the Double Bond

An alkene contains a carbon–carbon double bond, each of those carbons sp2 hybridised and trigonal planar, with rotation around the bond restricted because turning it would break the pi component. Ethene's carbons and hydrogens all lie in one plane. The restriction is local: a single bond elsewhere in the molecule still rotates freely.

Same side, or opposite.

Chem 2445 cis/trans Geometric Isomerism in Alkenes

Where an alkene carries two different groups on each double-bonded carbon, it can be cis, with like groups on the same side, or trans, with them opposite. Cis-two-butene has both methyls on one side; the trans form has them across. These are different compounds rather than one molecule seen from two angles, and the reason is the previous entry — free rotation happens at single bonds, not double ones.

Straight, and eager.

Chem 2448 Alkynes: Linear Triple Bonds

An alkyne contains a carbon–carbon triple bond, each of those carbons sp hybridised and linear, holding that section of the molecule at a rigid hundred and eighty degrees. Ethyne is fully linear for that reason. The extra pi bond makes an alkyne more reactive rather than more stable — alkynes undergo addition even more readily than alkenes do.

Three entries, and the first two are a registered twin — the pair this part is organised around. Whether a molecule has a multiple bond available decides which of two mechanisms it undergoes, and the two are distinguished by whether the carbon's hybridisation changes. The third entry is the case where the obvious prediction from a drawing turns out to be wrong.

Break the pi, make two bonds.

Chem 2451 Twin: The Addition Reaction

An addition reaction breaks the pi component of a double or triple bond and forms two new single bonds, one to each carbon — which changes those carbons' hybridisation. Bromine adds across ethene's double bond, taking the sp2 carbons to sp3 and giving one-two-dibromoethane. Addition needs an existing multiple bond to happen at, and it leaves the carbons hybridised differently than it found them.

Swap, and change nothing else.

Chem 2454 Twin: The Substitution Reaction

A substitution reaction replaces one atom or group on a saturated carbon with another, leaving the bond count and the hybridisation unchanged. Chlorine replaces a hydrogen on ethane to give chloroethane, the carbon sp3 throughout. Alkanes have no multiple bond to add across, so substitution is the mechanism available to them — expecting addition there misapplies what the previous entry described.

The drawing lies.

Chem 2457 Aromatic Delocalization Favors Substitution

Benzene's delocalised pi ring gives it unusual stability, so aromatic hydrocarbons undergo substitution rather than addition, which keeps the ring intact. Chlorine substitutes for one of benzene's hydrogens rather than adding across a double bond. Drawing benzene with alternating double bonds invites treating it as an alkene — its actual delocalised structure resists exactly the addition chemistry that drawing predicts.

Four entries and the part's second registered twin. From here on, the chain matters less than what is attached to it. An oxygen-hydrogen group makes an alcohol; a carbon double-bonded to oxygen makes either of two families depending on nothing more than a bonding count; and a combination of both makes the organic acids and the compounds formed from them.

Hydroxyl on a chain.

Chem 2460 Alcohols: The Hydroxyl Functional Group

An alcohol is a hydrocarbon in which a hydroxyl group replaces a hydrogen, named by swapping the alkane's final e for -ol plus a position number — propane's middle hydrogen replaced gives two-propanol. That hydroxyl is covalently bonded to carbon, unlike the hydroxide ion in sodium hydroxide, which is why alcohols are not strong bases despite sharing those two atoms.

Carbonyl at the end.

Chem 2463 Twin: The Aldehyde

An aldehyde contains a carbonyl group — carbon double-bonded to oxygen — whose carbon also bonds to at least one hydrogen, which places it at the end of a chain. The relationship to alcohols runs through the hydroxyl's position: a terminal hydroxyl, as in one-propanol, corresponds to the aldehyde propanal, with its carbonyl at the first carbon.

Carbonyl in the middle.

Chem 2466 Twin: The Ketone

A ketone contains a carbonyl whose carbon bonds to two other carbons rather than to a hydrogen, which places it inside the chain rather than at an end. Two-propanol's middle hydroxyl corresponds to propanone, better known as acetone. The carbonyl group itself is identical in both families — what separates an aldehyde from a ketone is only how many other carbons its carbonyl carbon is bonded to.

Both groups at once.

Chem 2469 Carboxylic Acids and Esters from the Carbonyl

A carboxylic acid's carbonyl carbon also carries a hydroxyl group, and that combination gives a weakly acidic hydrogen; an ester results when such an acid reacts with an alcohol, replacing that hydrogen. Acetic acid with ethanol gives ethyl acetate and water. The ionisable hydrogen sits on the hydroxyl oxygen rather than anywhere on the carbon chain, and that position alone accounts for the acidity.

Two entries on nitrogen's two families. Both centre on a nitrogen atom with a lone pair, and the difference between them is what sits next door — with a consequence for basicity that follows directly from it.

A lone pair to donate.

Chem 2472 Amines: Weak Bases from a Nitrogen Lone Pair

An amine contains a nitrogen bonded to carbons or hydrogens and carrying a lone pair, which makes amines weak bases in the same way ammonia is. Methylamine accepts a proton from water to form the methylammonium cation, exactly as ammonia does. The basicity comes from that lone pair — amines contain no hydroxide ion at all, so any resemblance to a hydroxide base is superficial.

Next to a carbonyl.

Chem 2475 Amides Distinguished from Amines by the Carbonyl

An amide's nitrogen bonds directly to a carbonyl carbon, where an amine's nitrogen bonds only to carbon or hydrogen. The peptide bond joining amino acids is an amide, formed between one molecule's carboxyl group and the next's amine. That neighbouring carbonyl draws electron density away from the nitrogen's lone pair, which makes amides weaker bases than amines — the lone pair is present but less available.

Two entries closing the part. First a shortcut: for organic molecules, the question of how oxidised a carbon is can be answered by counting two kinds of bond rather than running a full oxidation-number calculation. Then the capstone, which puts four of this part's families on one carbon backbone and predicts both their reactivity and their order along that oxidation scale.

Oxygen up, hydrogen down.

Chem 2478 Determining an Organic Carbon's Oxidation Level

Compare how many of a carbon's bonds go to oxygen against how many go to hydrogen: trading a carbon–hydrogen bond for a carbon–oxygen bond oxidises that carbon, and the reverse trade reduces it — which answers the question without a full oxidation-number calculation on a large molecule. Methane's carbon, bonded only to hydrogen, is fully reduced; replacing those bonds one at a time carries it through methanol, formaldehyde, formic acid and finally carbon dioxide, fully oxidised. The shortcut counts only bonds to hydrogen and to oxygen — applying it to bonds a carbon makes to other carbons misreads which bonds count.

One backbone, four families.

Chem 2483 Capstone: Functional-Group Family Comparison

Four compounds on the same five-carbon chain: pentane, one-pentanol, pentanal and pentanoic acid. Pentane is an alkane with no multiple bond, so substitution rather than addition is predicted — there is no pi bond to react across. One-pentanol carries its hydroxyl at a terminal carbon, so oxidising that carbon is predicted to give an aldehyde rather than a ketone, because the hydroxyl sits at the end of the chain and not its interior — and pentanal confirms it, its carbonyl carbon bonded to only one other carbon. Oxidising that carbon once more predicts a carboxylic acid, which pentanoic acid is. Across the series the functional carbon gains one carbon–oxygen bond at each step: none, then one, then two, then three. Reactivity class and oxidation position both follow from structure. One caution — pentanal is a point on a general sequence any similarly built alcohol passes through, not something reachable only from one-pentanol.

Five things to carry out of this part. A compound is organic when it is built around carbon bonding to itself — catenation — producing chains, branches and rings; but containing carbon is not sufficient, and oxides like carbon dioxide along with ionic compounds like the carbonates are conventionally excluded. Naming an alkane means finding the longest continuous chain for the base name, numbering from the end that gives the lowest substituent numbers, and listing substituents alphabetically — and where both directions give the identical set of numbers, the tie breaks alphabetically, with the substituent cited first taking the lower number. Alkenes undergo addition and alkanes undergo substitution because addition requires a multiple bond to react across: addition breaks a pi bond, forms two new single bonds and changes the carbons' hybridisation, while substitution swaps one group for another and leaves both bond count and hybridisation as they were. Benzene is the case worth remembering, because its delocalised ring makes it undergo substitution despite being drawn with double bonds. An aldehyde and a ketone contain the identical carbonyl group and are distinguished by one thing only — whether that carbonyl carbon bonds to one other carbon, putting it at a chain's end, or to two, putting it in the interior. And an organic carbon's oxidation level is found by counting its bonds to oxygen against its bonds to hydrogen, trading one for the other in either direction, which avoids a full oxidation-number calculation. Part Five closes this unit, and the course, with nuclear chemistry.