Question

Predict which of the following compounds will have a lower boiling point than${\text{1 - butanal}}$.
A.${\text{1 - butanol}}$
B.${\text{2 - butanol}}$
C. Butanoic acid
D. ${\text{1 - butene}}$

Answer 1

Hint: Boiling point of a liquid is the temperature at which the liquid boils and turns into a vapour. Greater the molecular weight of the compound will be its boiling point.

Complete step by step answer:
The boiling point is affected by the molecular weight of the compound i.e., As the molecular weight of the compound increases there will be increase in boiling point.
${\text{1 - butene}}$ has got a molecular weight lesser than that of ${\text{1 - butanal}}$. ${\text{1 - butanol}}$ contains ${\text{4C}}$ atoms and one${\text{O}}$ atom with a molecular weight of 74 ${\text{g/mol}}$ whereas the molecular weight of ${\text{1 - butene}}$ and butanoic acid is 56.11 ${\text{g/mol}}$ and 88.11${\text{g/mol}}$. ${\text{1 - butanol}}$, ${\text{2 - butanol}}$ and butanoic acid has got the ability to form intermolecular hydrogen bonding thus they will have high boiling point than ${\text{1 - butanal}}$ which doesn’t form any intermolecular hydrogen bond.
The correct answer is ${\text{1 - butene}}$ i.e D.

Additional information: If the 2 alkanes have got same molecular formula, then the alkane with more number of branching will have lower boiling point than the other this is because in straight chain the molecules are being packed tightly but in branching the molecule is prevented from being tightly packed.
Branching has got a reverse effect on melting point. Usually with the increase in the symmetry, melting point increases and forms a crystal lattice. In case of presence of the same number of carbon atoms increase in branching forms a more compact molecule hence the intermolecular attractive forces becomes stronger and results in higher melting point.

Note:
In case of alkenes and alkynes the melting point and boiling point increases with increase in molecular weight. The physical properties of alkenes and alkynes are similar to that of alkanes but the presence of double bond in alkenes causes cis/trans isomerisation affecting the polarity of the molecule, which in turn affect the intermolecular interactions.