Classification of Organic Compounds: MDCAT Chemistry notes

Organic chemistry: a starting point

Organic chemistry is the chemistry of carbon compounds. The idea that these compounds needed a "vital force" was overturned in 1828, when Friedrich Wöhler made urea by heating ammonium cyanate, an inorganic salt:

$$\mathrm{NH_4CNO \xrightarrow{\Delta} NH_2CONH_2}$$

Classification by carbon skeleton

1. Open-chain (acyclic or aliphatic) compounds have straight or branched chains, e.g. butane and isobutane.
2. Closed-chain (cyclic) compounds contain rings. These divide into:
   • Homocyclic (carbocyclic), where the ring is made of carbon atoms only. This group splits into two types:
     • Alicyclic: non-aromatic rings such as cyclopropane, cyclobutane and cyclohexane.
     • Aromatic: benzene and its derivatives (arenes), such as toluene and naphthalene.
   • Heterocyclic, where the ring contains at least one atom other than carbon (N, O or S).

Heterocyclic compounds

Compound	Ring size	Hetero atom	Formula
Pyridine	6	N	$\mathrm{C_5H_5N}$
Pyrrole	5	N	$\mathrm{C_4H_5N}$
Furan	5	O	$\mathrm{C_4H_4O}$
Thiophene	5	S	$\mathrm{C_4H_4S}$

Pyridine is benzene with one CH replaced by N. Pyrrole, furan and thiophene are five-membered heterocycles, not benzene derivatives.

Saturated and unsaturated compounds

• Saturated compounds have only C–C single bonds, so each of carbon's four valencies goes to a separate atom. Example: alkanes.
• Unsaturated compounds contain C=C or C≡C bonds, as in alkenes and alkynes.

Family	General formula
Alkanes	$\mathrm{C_nH_{2n+2}}$
Alkenes and cycloalkanes	$\mathrm{C_nH_{2n}}$
Alkynes	$\mathrm{C_nH_{2n-2}}$

Each ring costs two hydrogen atoms, so cycloalkanes have the same general formula as alkenes.

Naming

• Common and IUPAC names: formic acid is methanoic acid, acetic acid is ethanoic acid, propionic acid is propanoic acid and butyric acid is butanoic acid. Names ending "-anoic acid" are IUPAC names.
• $\mathrm{C_6H_5CH_2CH_2NH_2}$ is 2-phenylethanamine. The parent is ethanamine, with $\mathrm{NH_2}$ on C-1 and the phenyl group on C-2.
• In disubstituted benzenes, ortho, meta and para (1,2-, 1,3- and 1,4-) give the position of the second substituent relative to the first.

Classification by functional group

Organic compounds are also grouped by functional group into families such as alcohols, aldehydes, ketones, carboxylic acids, esters and amines. Each family forms a homologous series, and its members differ from one another by $\mathrm{CH_2}$.

Detecting elements

C and H are detected by combustion to $\mathrm{CO_2}$ and $\mathrm{H_2O}$. N, S and the halogens are detected by Lassaigne's (sodium fusion) test. Oxygen has no direct test; its percentage is found by difference.

Common MDCAT traps

• Homocyclic compounds divide into alicyclic and aromatic, not saturated and unsaturated.
• Pyridine is heterocyclic, not homocyclic. Its formula is $\mathrm{C_5H_5N}$, not $\mathrm{C_6H_5N}$.
• Cyclobutane is alicyclic, not aromatic.
• "Propionic" is the common name and "propanoic" the IUPAC name.
• Wöhler used ammonium cyanate, not ammonium carbonate.

Quick revision

• The cycloalkane formula is $\mathrm{C_nH_{2n}}$.
• Benzene ($\mathrm{C_6H_6}$) is the parent of the arenes.
• A heterocyclic ring contains N, O or S.
• Oxygen in an organic compound is estimated by difference.