Structural formulae

1. A structural formula is an unambiguous description of the way the atoms in a molecule are arranged.
2. It lists the atoms carbon by carbon along the chain, with each carbon written next to the atoms joined to it.
3. A molecular formula only counts the atoms, so it cannot tell you how they are joined.
   1. Ethanol has the molecular formula $C_2{H}_{6}O$, which does not show where the oxygen sits.
   2. Its structural formula, ${CH}_3{CH}_{2}OH$, shows a ${CH}_3$ group joined to a ${CH}_2$ group, which is joined to an $OH$ group.
4. A structural formula is shorter to write than a displayed formula, but it carries the same information about which atom is bonded to which.
5. Learn how to read three standard examples.
   1. ${CH}_2={CH}_2$ is ethene, and the $=$ sign must be written, because the double bond is the part that makes it an alkene.
   2. ${CH}_3{CH}_{2}OH$ is ethanol, with the $OH$ group on the end carbon.
   3. ${CH}_3{COOCH}_3$ is methyl ethanoate, an ester, in which the $COO$ group links a ${CH}_3$ group on each side.
6. In ${CH}_3{COOCH}_3$ the carbon of the $COO$ group has a double bond to one oxygen and a single bond to the other, and that second oxygen is bonded to the right-hand ${CH}_3$.
7. A branch on a chain is written in brackets after the carbon it is attached to, so ${CH}_{3}CH({CH}_3){CH}_3$ has a ${CH}_3$ branch on the second carbon.

Structural isomers

1. Structural isomers are compounds with the same molecular formula but different structural formulae.
2. Isomers contain exactly the same atoms, but the atoms are joined in a different order, so they are different compounds.
3. $C_4{H}_{10}$ has two structural isomers.
   1. Butane, ${CH}_3{CH}_2{CH}_2{CH}_3$, has all four carbon atoms in one unbranched chain.
   2. 2-methylpropane, ${CH}_{3}CH({CH}_3){CH}_3$, has a chain of three carbon atoms with a ${CH}_3$ branch on the middle carbon.
4. $C_4{H}_8$ also has structural isomers, which differ in the position of the double bond.
   1. But-1-ene, ${CH}_3{CH}_{2}CH={CH}_2$, has the $C=C$ bond at the end of the chain.
   2. But-2-ene, ${CH}_{3}CH={CHCH}_3$, has the $C=C$ bond in the middle of the chain.
5. To decide whether two structures are isomers, carry out two checks in order.
   1. Count each type of atom in both structures and confirm that the molecular formulae are the same.
   2. Trace the longest carbon chain and the positions of any branches or double bonds to confirm that the atoms are joined differently.
6. A chain drawn with a bend in it is still the same compound, because bending the drawing does not change which atoms are bonded to each other.
7. To draw an isomer of a given compound, shorten the main chain and move one carbon into a branch, or move a double bond to a new position.

Features of a homologous series

1. A homologous series is a family of similar compounds with similar chemical properties due to the presence of the same functional group.
2. Every homologous series has five general characteristics.
3. Same functional group
   1. All members carry the same reactive atom or group, such as $C=C$ in the alkenes or $-OH$ in the alcohols.
4. Same general formula
   1. All members fit one formula in $n$, such as $C_n{H}_{2n+2}$ for the alkanes.
5. Differ by a $-{CH}_2-$ unit
   1. Each member has one more carbon atom and two more hydrogen atoms than the member before it.
   2. Methane, ${CH}_4$, ethane, $C_2{H}_6$, and propane, $C_3{H}_8$, each differ from the next by ${CH}_2$.
   3. The relative molecular mass therefore goes up by 14 from one member to the next.
6. Trend in physical properties
   1. Physical properties such as boiling point change gradually as the chain gets longer.
7. Similar chemical properties
   1. All members react in the same kinds of way, because the reactions happen at the shared functional group.

The boiling point trend

1. Along a homologous series, boiling point increases as the number of carbon atoms increases.
2. The explanation follows a chain of reasoning.
   1. A longer chain means a larger molecule with more atoms.
   2. Larger molecules have stronger intermolecular forces, the weak forces of attraction between neighbouring molecules.
   3. More energy is needed to overcome these stronger forces when the liquid boils.
   4. So the liquid must be heated to a higher temperature before it boils.
3. Boiling separates whole molecules from each other, so the covalent bonds inside each molecule are not broken.
4. The trend also shows up as a change of state, since the smallest alkanes are gases at room temperature, the middle members are liquids and the longest are solids.