Problem 62 - Chemistry and Chemical Reactivity

Question

In which of the following organic molecules might we expect hydrogen bonding to occur? (a) methyl acetate, \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{CH}_{3}\) (b) acetaldehyde (ethanal), \(\mathrm{CH}_{3} \mathrm{CHO}\) (c) acetone (2-propanone) (see Question 8) (d) benzoic acid \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CO}_{2} \mathrm{H}\right)\) (e) acetamide (CH \(_{3} \mathrm{CONH}_{2}\) an amide formed from acetic acid and ammonia) (f) \(\mathrm{N}, \mathrm{N}\) -dimethylacetamide \(\left[\mathrm{CH}_{3} \mathrm{CON}\left(\mathrm{CH}_{3}\right)_{2}\right.\),an amide formed from acetic acid and dimethylamine]

Step-by-Step Solution

Answer

Benzoic acid and acetamide can form hydrogen bonds.

Step 1: Understanding Hydrogen Bonding

Hydrogen bonding is an intermolecular force that occurs in molecules containing hydrogen atoms bonded to highly electronegative atoms like nitrogen (N), oxygen (O), or fluorine (F). This bonding typically occurs when there is a N-H, O-H, or F-H group in the molecule.

Step 2: Analyze Methyl Acetate

Methyl acetate (\(\mathrm{CH}_3\mathrm{CO}_2\mathrm{CH}_3\)) does not contain any N-H, O-H, or F-H groups. Hence, it cannot form hydrogen bonds.

Step 3: Analyze Acetaldehyde (Ethanal)

Acetaldehyde (\(\mathrm{CH}_3\mathrm{CHO}\)) contains a C=O bond but no O-H, N-H, or F-H groups, so it cannot participate in hydrogen bonding.

Step 4: Analyze Acetone

Acetone (2-propanone) has a C=O group but lacks O-H, N-H, or F-H groups needed for hydrogen bonding. Hence, it does not exhibit hydrogen bonding.

Step 5: Analyze Benzoic Acid

Benzoic acid (\(\mathrm{C}_6\mathrm{H}_5\mathrm{CO}_2\mathrm{H}\)) contains a -COOH group, which has an O-H bond. This O-H bond can form hydrogen bonds with other molecules, enabling hydrogen bonding.

Step 6: Analyze Acetamide

Acetamide (\(\mathrm{CH}_3\mathrm{CONH}_2\)) has an N-H bond in the amide group, which can take part in hydrogen bonding due to the presence of the electronegative nitrogen atom.

Step 7: Analyze N, N-Dimethylacetamide

N, N-Dimethylacetamide \([\mathrm{CH}_3\mathrm{CON}(\mathrm{CH}_3)_2]\) lacks the N-H bond due to methyl substitution on the nitrogen atom, which means it cannot participate in hydrogen bonding.

Key Concepts

Organic Molecules

Organic molecules are compounds that primarily consist of carbon atoms bonded with hydrogen, oxygen, nitrogen, and sometimes other elements. The backbone of most organic compounds is made up of carbon atoms connected to one another and to other atoms by covalent bonds. These structures can vary greatly, leading to a wide variety of properties and functions.
One key feature of organic molecules is the presence of functional groups. Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. For instance, the amide group, carboxyl group, and hydroxy group are all functional groups commonly found in organic molecules.
Notable organic compounds include alcohols, acids, esters, and amides, each contributing different properties to the compounds they form part of. Organic chemistry studies these compounds and their reactions, revealing insights into everything from biological processes to synthetic polymer creation.

Intermolecular Forces

Intermolecular forces are the forces that mediate interaction between molecules, including forces of attraction or repulsion. These forces are critical in determining the physical properties of substances, such as boiling and melting points, solubility, and density.
Three primary types of intermolecular forces include:

• London Dispersion Forces: Present in all molecules, they are the weakest form of intermolecular forces, resulting from temporary dipoles created when electron clouds shift within molecules.
• Dipole-Dipole Interactions: Occur in polar molecules where positive and negative charges attract each other.
• Hydrogen Bonding: A stronger type of dipole-dipole interaction that occurs when hydrogen is bonded to very electronegative atoms like nitrogen, oxygen, or fluorine, which is crucial in many biological structures and synthetic materials.

Understanding these forces allows us to predict how different substances will behave in various conditions and why certain molecules interact as they do.

Electronegative Atoms

Electronegativity is a measure of an atom's ability to attract and hold electrons. Atoms with higher electronegativity tend to attract electrons more strongly, enabling the formation of polar covalent bonds.
In organic chemistry, electronegative atoms are crucial because they influence the polarity of molecules. The most common electronegative atoms involved in hydrogen bonding are nitrogen (N), oxygen (O), and fluorine (F). These atoms form highly polarized bonds with hydrogen, creating partial positive and negative charges which facilitate hydrogen bonding.
For example:

• Nitrogen: Often found in amides and nitriles, it can establish hydrogen bonds due to its lone pairs and high electronegativity.
• Oxygen: Key in alcohols, acids, and carbonyl compounds, capable of forming hydrogen bonds due to its two lone pairs.
• Fluorine: The most electronegative, found in some specialized molecules, forming very strong hydrogen bonds.

This polar characteristic leads to significant effects on molecule solubility, boiling points, and interaction with other entities.

Amide Group

The amide group is a functional group characterized by a carbonyl group (\[ C=O \]) bonded to a nitrogen atom (\[ N \]). This arrangement is common in organic compounds, particularly in proteins where they form peptide bonds, linking amino acids together.
The structure of the amide group enables specific interactions, particularly through hydrogen bonding. Although it contains a highly polarized bond due to the presence of the electronegative nitrogen, it may or may not enable hydrogen bonding depending on whether the nitrogen's hydrogen is substituted.

• Non-substituted Amides: These retain an N-H bond, allowing for effective hydrogen bonding, as seen in compounds like acetamide.
• Substituted Amides: When hydrogen atoms on the nitrogen are replaced, such as in N,N-dimethylacetamide, the ability to form hydrogen bonds is lost because there is no N-H bond available.

The presence of the amide group imparts stability to molecular structures and influences boiling points and solubility due to its capability to engage in dipole-dipole interactions and, where relevant, hydrogen bonding.