Propanoic acid, C.HCOOH, is an organic acid that is a liquid at room temperature.
(a) An incomplete Lewis diagram for the propanoic acid molecule is provided in the box below. Complete the diagram, showing how the remaining atoms in the molecule are arranged around the carbon atom marked with an asterisk (*). Your structure should minimize formal charge and include any lone pairs of electrons.
(b) Identify the hybridization of the carbon atom marked with the asterisk
(c) Propanoic acid has a lower boiling point than butanoic acid, CH.COOH.
Identify all the types of intermolecular forces present among the molecules in propanoic acid.
Which of the types of intermolecular forces that you identified in part
(c)(i) is most responsible for the difference in boiling points of the two acids?
The Correct Answer and Explanation is :
Sure, let’s address each part of your question step by step.
(a) Complete Lewis Diagram for Propanoic Acid
First, let’s clarify the structure of propanoic acid (C3H6O2). The molecular formula for propanoic acid can be written as CH3CH2COOH. Here’s a step-by-step guide on how to complete the Lewis diagram:
- Base Structure: The base structure includes a backbone of three carbon atoms. The COOH (carboxylic acid) group is typically at one end.
- Carbon Atoms:
- The first carbon (from the left) in the CH3 group is bonded to three hydrogen atoms.
- The second carbon, marked with an asterisk (*), is part of the CH2 group and is bonded to two hydrogen atoms.
- The third carbon is part of the COOH group. It is double bonded to an oxygen (O=) and single bonded to another oxygen (OH).
- Oxygen Atoms:
- The double-bonded oxygen in the COOH group has two lone pairs of electrons.
- The hydroxyl oxygen (OH) has two lone pairs and is single bonded to a hydrogen.
(b) Hybridization of the Carbon Atom Marked with an Asterisk (*)
The carbon atom marked with an asterisk in CH3CH2COOH is the second carbon in the chain, part of the CH2 group. This carbon is sp3 hybridized because it forms four sigma bonds (two with hydrogen atoms and two with adjacent carbon atoms), with no pi bonds.
(c) Intermolecular Forces in Propanoic Acid
Propanoic acid molecules exhibit the following types of intermolecular forces:
- London Dispersion Forces: These are present due to the temporary dipoles in the molecule, influenced by the electron distribution within and among all atoms.
- Dipole-Dipole Interactions: Because the COOH group is polar, these forces occur between the positively charged hydrogen of one molecule and the negatively charged oxygen of another.
- Hydrogen Bonding: The strongest of the intermolecular forces in propanoic acid, this occurs specifically due to the hydrogen of the hydroxyl group (OH) in COOH bonding to the oxygen of the COOH group in another molecule.
(c)(i) Responsible Force for Boiling Point Difference with Butanoic Acid
Hydrogen bonding is the most significant intermolecular force affecting the boiling points of propanoic acid (C3H6O2) and butanoic acid (C4H8O2). Butanoic acid, with one extra carbon, has a slightly higher molecular weight and a larger surface area than propanoic acid. This increases the London dispersion forces in butanoic acid, making them stronger compared to those in propanoic acid. However, both acids engage in hydrogen bonding, which is pivotal but similar in strength in both cases. The difference in boiling points primarily results from the stronger London dispersion forces in butanoic acid due to its increased molecular size and surface area.