Acetic Anhydride Formula, Density & Uses

Maram Ghadban, Erin Monagan
  • Author
    Maram Ghadban

    A freelance tutor equipped with a bachelor's of science in chemical engineering. Graduated from the American University of the Middle East with a GPA of 3.87, performed a number of scientific primary and secondary research. Tutored university level students in various courses in chemical engineering, math, and art. Has experience tutoring middle school and high school level students in science courses.

  • Instructor
    Erin Monagan

    Erin has been writing and editing for several years and has a master's degree in fiction writing.

Learn about acetic anhydride and its formula. Understand properties of acetic anhydride such as density, molecular weight, boiling point, and its common uses. Updated: 12/28/2021

What Is Acetic Anhydride?

Acetic anhydride is an organic compound that is composed of carbons, hydrogens, and oxygens. This organic molecule is the simplest carboxylic acid anhydride, which is a functional group that consist of an oxygen atom that is cornered by two acyl groups (RC=O). Acetic anhydride is commonly used as a reagent in the synthesis of various organic compounds. It is also used as an ingredient in many pharmaceutical products that consumers use, like aspirin and acetaminophen.

Figure 1: An acyl group is composed of a side chain R bonding to a carbonyl end C=O

Acyl group in anhydrides

The carboxylic acid, ethanoic acid, is the primary precursor of acetic anhydride; eliminating the water content in two ethanoic acids results to them fusing to form acetic anhydride. The following list summarizes a few of acetic anhydride's alternative names:

  • Ethanoic anhydride is the systematic name the International Union of Pure and Applied Chemistry (IUPAC) has set. Acetic anhydride is basically two ethanoic acids fused together after dehydrating them.
  • Acetyl oxide.
  • Its identifier number in the Chemical Abstract Services (CAS) is 108-24-7.

What Is Acetic Anhydride?

When you have a nasty headache, I am sure one of the first medicines you may reach for is a bottle of aspirin. Did you know that acetic anhydride is a reagent used to make aspirin? Besides its many uses, acetic anhydride has a very distinct background and properties.

Acetic anhydride is an organic compound also known as a type of acid anhydride. The molecular formula for acetic anhydride is C4H6O3. Don't be alarmed if you hear scientists refer to acetic anhydride as ethanoic anhydride. Ethanoic anhydride is just another name for acetic anhydride.

The synthesis of acetic anhydride is quite simple as long as you remember the term carboxylic acid. A carboxylic acid is a functional group whose molecular formula is defined as RCOOH. Can you spot the carboxylic acid in the structure of ethanoic acid? Now, I know you may be wondering why you need to spot this functional group, let alone look at the structure of this organic acid. Well, this acid is a building block, or precursor, towards making acetic anhydride.

ethanoic acid

When there are two ethanoic acid molecules present, removing a water molecule results in the production of ethanoic anhydride, also known as acetic anhydride. Essentially, you are performing a dehydration reaction. That is, you are dehydrating or quenching all of the water from the ethanoic acid to form ethanoic anhydride.

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Acetic Anhydride Formula and Structure

The acetic anhydride formula is {eq}C_4H_6O_3 {/eq}; this compound is made of four carbons, six hydrogens and three oxygens. An alternative way of writing the chemical formula for acetic anhydride is {eq}(CH_3CO)_2O {/eq}, it is also abbreviated as {eq}Ac_2O {/eq}. As mentioned earlier, the functional group of acetic anhydrides is the anhydride group, which is composed of two acyl groups attached to an oxygen atom, as shown in figure 2.

Figure 2: Anhydride is a functional group where two acyl groups are attached to the same oxygen

Acetic anhydride functional group

The structure of acetic anhydride is presented in figures 3 and 4. The structure shows two acyl groups {eq}CH_3CO {/eq} attached to an oxygen atom in the center.

Figure 3: The 2D structure of acetic anhydride

Acetic anhydride structure

Figure 4: The 3D structure of acetic anhydride

Acetic anhydride structure

Properties of Acetic Anhydride

Acetic anhydride is a colorless liquid with a strong odor and a sour taste. Acetic anhydride density is quite high; it is slightly higher than the density of water (0.999 g/mL), which makes it heavier than water. The acetic anhydride boiling point is relatively high; it is estimated to be equal to 139.5{eq}^{\circ} {/eq}C. The acetic anhydride molecular weight and other important properties are documented in the table.

Parameter Data
Physical State Liquid
Molecular Weight 102.089 g/mol
Density 1.072g/mL
Solubility in Water 2.6 g/100 mL
Flammability Flammable
Toxicity Toxic
Vapor Pressure 5.94mmHg at 25C
Melting Point -73.1C
Boiling Point 141.1C
Odor Strong vinegar-like odor
Taste Sour
Color Colorless
Flash Point 54.4C
Autoignition Temperature 316C
Refractive Index 1.386
Corrosiveness Corrosive
pKa Value -6.9

Only 2.5 grams of acetic acid is dissolved when placed in 100 milliliters of water; acetic anhydride undergoes hydrolysis when left to react with water. The hydrolysis of acetic anhydride involves the formation of acetic acid, which is a carboxylic acid, as shown:

{eq}(CH_3CO)_2O + H_2O \rightarrow 2CH_3CO_2H {/eq}

Acetic acid is fully miscible in water; it dissolves completely in water. The pKa value of acetic acid is lower than zero, this shows that acetic acid is a strong acid. As a rule, the lower the pKa value the stronger the acid. Strong acids have pKa values lower than zero.

Production of Acetic Anhydride

There are various methods of producing acetic anhydride; the following subsections summarize a few of the important production methods.

From The Carbonylation of Methyl Acetate

Acetic anhydride can be synthesized by reacting methyl acetate with carbon monoxide in a carbonylation reaction, as shown:

{eq}CH_3CO_2CH_3 + CO \rightarrow (CH_3CO)_2O {/eq}

Properties of Acetic Anhydride

Acetic anhydride is known to be a colorless liquid that has a highly offensive smell. Often, scientists characterize this pungent smell as akin to vinegar. The molecular weight of acetic anhydride is 102.1 g/mol. It boils at a temperature 139C and melts at a temperature of -73C. The density of acetic anhydride is 1.08g/cm3. Regarding reactivity, acetic anhydride loves to react with water and alcohol. It especially likes to react with water, which is known as a hydrolysis reaction. This will lead to the formation of acetic acid, also known as vinegar. Besides being reactive, it is also characterized as a combustible compound. This simply means that, under specific conditions, acetic anhydride can become ignitable and create a fire hazard.

It is worth noting acetic anhydride does have toxic properties. This must be considered prior to the handling and use of this compound. If you inhale it, or breath it in, it can be toxic. This compound is a lachrymator, which means it hurts your eyes and makes them water. It is also toxic if ingested. Given these properties, handling of acetic anhydride (and reactions involving this compound) is typically performed in a well-ventilated laboratory hood.

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Video Transcript

What Is Acetic Anhydride?

When you have a nasty headache, I am sure one of the first medicines you may reach for is a bottle of aspirin. Did you know that acetic anhydride is a reagent used to make aspirin? Besides its many uses, acetic anhydride has a very distinct background and properties.

Acetic anhydride is an organic compound also known as a type of acid anhydride. The molecular formula for acetic anhydride is C4H6O3. Don't be alarmed if you hear scientists refer to acetic anhydride as ethanoic anhydride. Ethanoic anhydride is just another name for acetic anhydride.

The synthesis of acetic anhydride is quite simple as long as you remember the term carboxylic acid. A carboxylic acid is a functional group whose molecular formula is defined as RCOOH. Can you spot the carboxylic acid in the structure of ethanoic acid? Now, I know you may be wondering why you need to spot this functional group, let alone look at the structure of this organic acid. Well, this acid is a building block, or precursor, towards making acetic anhydride.

ethanoic acid

When there are two ethanoic acid molecules present, removing a water molecule results in the production of ethanoic anhydride, also known as acetic anhydride. Essentially, you are performing a dehydration reaction. That is, you are dehydrating or quenching all of the water from the ethanoic acid to form ethanoic anhydride.

Properties of Acetic Anhydride

Acetic anhydride is known to be a colorless liquid that has a highly offensive smell. Often, scientists characterize this pungent smell as akin to vinegar. The molecular weight of acetic anhydride is 102.1 g/mol. It boils at a temperature 139C and melts at a temperature of -73C. The density of acetic anhydride is 1.08g/cm3. Regarding reactivity, acetic anhydride loves to react with water and alcohol. It especially likes to react with water, which is known as a hydrolysis reaction. This will lead to the formation of acetic acid, also known as vinegar. Besides being reactive, it is also characterized as a combustible compound. This simply means that, under specific conditions, acetic anhydride can become ignitable and create a fire hazard.

It is worth noting acetic anhydride does have toxic properties. This must be considered prior to the handling and use of this compound. If you inhale it, or breath it in, it can be toxic. This compound is a lachrymator, which means it hurts your eyes and makes them water. It is also toxic if ingested. Given these properties, handling of acetic anhydride (and reactions involving this compound) is typically performed in a well-ventilated laboratory hood.

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Frequently Asked Questions

How do you make acetic anhydride from acetic acid?

Two acetic acid molecules are subjected to high temperatures where their water content is removed. The -OH from one acetic acid and the H from the other acid are eliminated. The remaining constituents of both acids fuse to form acetic acid. This is done in the dehydration reaction.

What is the density of acetic anhydride in g mL?

The density of acetic anhydride is estimated to be equal to 1.072g/mL. Its density is slightly larger than water's, whose density is equal to 0.999 g/mL. Acetic anhydride is heavier than water.

What state is acetic anhydride?

Acetic anhydride exists in the liquid state at normal conditions; at room temperature and atmospheric pressure. It is a highly flammable colorless liquid that has a vinegar like odor.

Is acetic anhydride a strong acid?

Acetic anhydride is a relatively strong acid; its pKa value is -6.9, it is less than zero. Strong acids have pKa values lower than zero. The lower the pKa value the stronger the acid.

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