Functional groups: classification of organic compounds

The chemistry of **carbon compounds**.

A family of organic compounds with the **same general formula** and **functional group**, each member differing by **CH_{2}**.

The reactive atom or group of atoms that gives a series its **characteristic chemistry** (e.g. C=C, –OH).

Same **general formula**; differ by **CH_{2}**; **gradual change** in physical properties; **similar chemical** properties.

**C_{n}H_{2n+2}** (saturated — only single C–C bonds).

**C_{n}H_{2n}** (unsaturated — one C=C double bond).

**C_{n}H_{2n+1}OH** (functional group –OH).

**Saturated** = only single C–C bonds (max H); **unsaturated** = at least one **C=C** double bond (fewer H).

Longer chains are bigger/heavier, so **intermolecular forces** are stronger → **higher boiling point**.

**Two** fewer hydrogens in the alkene — the C=C double bond replaces two C–H bonds.

**Ethene, C_{2}H_{4}** (alkenes start at n = 2).

The **simplest whole-number ratio** of the atoms in a compound (e.g. CH_{2}O for glucose).

The **actual number** of each type of atom in one molecule (e.g. C_{6}H_{12}O_{6} for glucose).

A diagram showing **every atom and every bond** in the molecule.

Atoms written **grouped in a line** with the bonds implied (e.g. CH_{3}CH_{2}OH).

Only the **carbon skeleton** drawn as lines; carbons are corners/ends and **H on carbon is implied**; functional groups are shown.

A **carbon** atom (each with enough H to make four bonds, not drawn).

Divide **every** subscript by their **highest common factor** (e.g. C_{6}H_{12}O_{6} ÷ 6 = CH_{2}O).

Molecules with the **same molecular formula** but a **different arrangement** of atoms (different connectivity).

Chain **branching**, **position** of a group, or different **functional group / class**.

Keep the **same molecular formula** but **change the connectivity** — never just rotate or flip the original.

No — CH_{2}O is an **empirical** formula; C_{2}H_{4}O_{2} (ethanoic acid) is one **molecular** formula with that ratio.

**No** — it is the same molecule; an isomer must have a genuinely different structure.

The **reactive atom or group of atoms** that defines an organic molecule's class and chemistry.

A family of compounds with the **same functional group** and the same general formula.

Saturated = only single C–C bonds (alkane); unsaturated = has a **C=C** (or C≡C) bond (alkene).

**–OH** (hydroxyl); name ends in **-ol** (e.g. propan-1-ol).

**–COOH** (carboxyl); name ends in **-oic acid** (e.g. propanoic acid).

Both have C=O. Aldehyde = carbonyl at the **end** (-al); ketone = carbonyl in the **middle** (-one).

An alkane with a **halogen** (–F, –Cl, –Br, –I) in place of an H; named with a prefix (chloro-, bromo-…).

**-ene**, because it contains a **C=C** double bond (e.g. propene).

**Stem** (number of carbons) + **suffix** (functional group) + **locant** (where the group is).

1 = meth-, 2 = eth-, 3 = prop-, 4 = but-.

The **number** in a name showing the position of the functional group on the chain (e.g. the 2 in but-2-ene).

Give the functional group the **lowest possible locant**.

Its m/z value is the **relative molecular mass (Mr)** — M⁺ is the peak at the **highest** m/z.

The **mass lost** (M⁺ − fragment) shows which **group broke off** (e.g. loss of 15 = CH_{3}).

Loss of a **CH_{3}** (methyl) group.

Loss of an **OH** group.

The **functional group**, from a characteristic absorption **wavenumber** (cm⁻¹) given in the data booklet.

An **O–H** group (an alcohol). A carboxylic acid O–H is even broader, ~2500–3000.

A **C=O** (carbonyl) — aldehyde, ketone, acid or ester.

The **number of peaks = number of different hydrogen environments** in the molecule.

**Three** — the CH_{3}, CH_{2} and OH hydrogens are three different environments.

By **symmetry** the two CH_{3} groups are equivalent, so all six H are in one environment.

**MS** (Mr + fragments), **IR** (functional group), **¹H NMR** (number of H environments) — used together.

In the **data booklet** — you read the wavenumbers off, you don't memorise them.

Different compounds with the **same molecular formula** but a different arrangement of atoms.

**Structural** isomers differ in **connectivity** (which atom bonds to which); **stereoisomers** have the same connectivity but a different **arrangement in space**.

**Chain** (different carbon skeleton), **position** (group in a different place), and **functional-group** (a different functional group/family).

Ethanol (CH_{3}CH_{2}OH, an alcohol) and methoxymethane (CH_{3}OCH_{3}, an ether) — both C_{2}H_{6}O.

A **C=C double bond** (restricted rotation) **and** two **different** groups on **each** doubly-bonded carbon.

**cis** = the two like groups on the **same** side of the C=C; **trans** = on **opposite** sides.

But-2-ene has a CH_{3} and an H on each C=C carbon (two different groups each); but-1-ene has two identical H atoms on one carbon, so no isomers.

**Priority** by Cahn–Ingold–Prelog: the atom of **higher atomic number** is higher priority. Z = higher-priority groups on the same side; E = on opposite sides.

A carbon bonded to **four different** atoms or groups (a stereocentre, often marked C*).

The **two non-superimposable mirror-image** forms of a molecule that has a chiral carbon (optical isomers).

They **rotate plane-polarised light** by the same angle in **opposite directions** (one +, one −); all their other physical properties are identical.

A **50:50 mixture** of the two enantiomers, which shows **no net rotation** of plane-polarised light (the effects cancel).

**sp²** — three σ bonds in a plane at **120°**, leaving one electron in a **p-orbital** perpendicular to the ring.

A **planar (flat) regular hexagon** of six carbons, each bonded to one hydrogen.

Two ring-shaped electron clouds **above and below** the ring, formed by sideways overlap of the six carbon p-orbitals.

**All six are equal** (~140 pm) — **intermediate** between a single (~154 pm) and a double bond (~134 pm).

**(1)** All six C–C bonds are the **same length**. **(2)** The **enthalpy of hydrogenation** is **less negative** than expected for three isolated C=C bonds.

Real benzene is **lower in energy (more stable)** than a Kekulé structure — the difference is the **delocalisation (resonance) stabilisation**.

**Kekulé** = alternating single/double bonds (two bond lengths). **Delocalised** = a circle in the hexagon (all bonds equal) — the **accepted** model.

Addition would **break the delocalised π system** and lose its stability, so benzene avoids it.

**Substitution** — a ring H is replaced (usually with a catalyst) and the **delocalised ring stays intact**.

Micro **6.4.3** (electrophilic substitution — the curly-arrow mechanism); 3.2.6 only sets it up qualitatively.

**(1)** add HX (e.g. HBr) → halogenoalkane; **(2)** warm with aqueous NaOH → alcohol (–X replaced by –OH).

**Orange → green** (Cr_{2}O_{7}^{2-} is reduced to Cr^{3+}).