Acid Chloride: Formation & Reaction

Instructor: Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

Acid chlorides are compounds created from a very interesting reaction. Explore acid chlorides: learn about the mechanism required to perform this reaction and their role in the formation of anhydrides.

Overview: What Are Acid Chlorides?

Before we get started, looking at the structure below, can you guess what this organic compound is? It is an acid chloride. Specifically, this structure is for the compound acetyl chloride. Acid chlorides are organic compounds composed of a chlorine atom attached to an acyl group.

Molecular Structure of Acetyl Chloride

An acyl group is a functional group defined by the molecular formula of RCO. Functional groups are portions of a molecule known to be a recognized group of bonded atoms. Think of an R group as any molecule or atom you can attach to the carbon atom in RCO. Examples of different acyl groups, highlighting the R group is shown.

Examples of Different Acyl Groups

Going back to our friend, acetyl chloride, can you spot the acyl group and chlorine atom? Both are highlighted in the following figure shown. Always remember, the presence of a chlorine atom and an acyl group is a great hint that a compound may belong to the acid chloride family.

Highlight of Acyl Group and Chlorine Atom in Acetyl Chloride

The molecular formula for acid chloride is RCOCl. Did you notice the similarity between the acyl and acid chloride formula? That's right, they both have R groups. However, the placement of these R groups are different. In the case of acid chloride, the R group is attached to the COCl group.

The acid chloride family belongs to a larger organic family called the acyl halides. Acyl halides are organic compounds that have an acyl group attached to a halide group. Think of the word halide as a fancy name for chemical elements belonging to the halogen family on a periodic table.

Unique Properties

One property of an acid chloride is its aversion to forming hydrogen bonds with other compounds. This lack of hydrogen bond formation affects their boiling and melting point. A hydrogen bond is a chemical bond formed when a hydrogen atom, in one molecule, is attracted to an electronegative atom on a different molecule. Electronegative. simply refers to an atom that absolutely loves to attract electrons to form a chemical bond.

If we compare the acid chloride family to carboxylic acids, we will see the boiling/melting point of acid chloride is lower than a carboxylic acid. Just as a refresher, a carboxylic acid is an organic compound whose molecular formula is RCOOH. For example, acetyl chloride has a boiling point of approximately 51C. This is much lower than acetic acid's boiling point of 118C, which is a carboxylic acid.

Acid Chloride Formation Using Thionyl Chloride

There is a distinct mechanism to make acid chlorides using thionyl chloride compounds. This mechanism, the process used to complete an acid chloride reaction, is called nucleophilic acyl substitution.

General Reaction of Acid Chloride Formation

Let's break down the steps occurring in this reaction.

Step 1: We call the oxygen atom, in the alcohol group, a nucleophile. A nucleophile is a molecule or ion that will donate its electron pair to form a chemical bond. This nucleophile attacks the carbon atom, an electrophile, in the carbonyl group (CO). An electrophile is a molecule that accepts an electron pair to form a chemical bond. When the nucleophile attacks the electrophile, the end result is an intermediate formed with our friend thionyl chloride.

Nucleophilic Attack and Formation of Tetrahedral Intermediate

Step 2: Following this attack, the intermediate collapses. This causes the removal of the chlorine atom as a leaving group from thionyl chloride. A leaving group is any fragment (molecule or atom) that leaves with its electrons. Once the leaving group is removed, a chlorosulfite ion is formed.

Removal of Chlorine Leaving Group and Formation of Chlorosulfite Ion

Step 3: A second nucleophilic attack on the carbonyl group of the chlorosulfite ion occurs. The chlorine atom, a leaving group, bonds to the carbon atom. Note that this carbon atom is an electrophile. Once bonded, an intermediate is formed.

Second Nucleophilic Attack to Form Tetrahedral Intermediate

Step 4: The leaving group is removed and intermediate collapses. This leads to the production of yet another intermediate.

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