Structural isomerism

Structural isomers have the same molecular formula (the same atoms) but the atoms are joined in a different order, so the structural formulae differ: for example butane and methylpropane, both C₄H₁₀. Same structural formula with a different arrangement in space describes stereoisomers (cis-trans and optical). Same empirical formula with different molecular formulae, such as ethene and propene (both CH₂), are not isomers at all, because they contain different numbers of atoms. Members of a homologous series differ by CH₂, so they have different molecular formulae.

Only two structures are possible: butane, CH₃CH₂CH₂CH₃, and methylpropane, (CH₃)₃CH. Answers of 3 or 4 usually come from drawing the same molecule twice, for example a bent chain or 1-methylpropane, which is just butane drawn with a kink. 1 ignores the branched chain isomer.

Both have the same unbranched four-carbon chain and the same functional group, OH; only the position of the OH changes (C1 versus C2). That is positional isomerism. Chain isomers differ in the carbon skeleton (straight or branched), which is the same here. Functional group isomers contain different functional groups, but both are alcohols. Cis-trans isomerism is a type of stereoisomerism that needs a C=C, and these compounds have different structural formulae, so they are structural isomers.

Propanal is an aldehyde and propanone is a ketone, so the same molecular formula gives two different functional groups: functional group isomerism. Positional isomers keep the same functional group and move it along the chain; moving C=O from the end to the middle changes aldehyde to ketone, so this is classed as a change of functional group. Both have an unbranched three-carbon chain, so it is not chain isomerism. Optical isomerism needs a chiral carbon with four different groups; neither compound has one.

The three isomers are pentane, 2-methylbutane and 2,2-dimethylpropane. They differ only in the carbon skeleton, so this is chain isomerism. 4 counts 3-methylbutane as a separate compound, but it is 2-methylbutane numbered from the wrong end. Alkanes have no functional group to move, so positional isomerism does not apply, and they cannot show functional group isomerism; 2 also misses an isomer.

On the butane chain Br can go on an end carbon (1-bromobutane) or a middle carbon (2-bromobutane). On the methylpropane skeleton Br can go on a CH₃ carbon (1-bromo-2-methylpropane) or on the central carbon (2-bromo-2-methylpropane). That gives 4. 2 counts only the positions on the straight chain. 3 misses one of the branched isomers. 5 counts a duplicate, such as 3-bromobutane or 4-bromobutane, which are the same molecules as 2-bromobutane and 1-bromobutane numbered from the other end.

With OH, the group can be on an end carbon (propan-1-ol) or the middle carbon (propan-2-ol): positional isomers. The O can also link two carbon groups, giving the ether CH₃OCH₂CH₃, methoxyethane, a functional group isomer. Total 3. Stopping at 2 forgets the ether. Propan-3-ol is propan-1-ol numbered from the wrong end, not a new compound. Ethoxymethane is the same molecule as methoxyethane written the other way round.

Methyl methanoate, HCOOCH₃, is C₂H₄O₂, the same as ethanoic acid, but it is an ester rather than a carboxylic acid: a functional group isomer. Methyl ethanoate is C₃H₆O₂, one CH₂ more, so it is not an isomer. Ethanal is C₂H₄O, one O short. Ethane-1,2-diol is C₂H₆O₂, with two extra H atoms, so it is not an isomer either.