An ester is a chemical that is formed when an organic acid reacts with an alcohol.  Esters frequently have distinctive odors, and are found in the flavorings of many fruits and plants. The general form of a reaction between an organic acid and an alcohol is shown below:

 Organic Acid + Alcohol                 Ester             +    Water

In the formulas above, R and R’ represent general groups of hydrocarbons.  The circled groups on the acid and alcohol combine to form water, and the pieces left behind form the ester.  A little bit of concentrated sulfuric acid is added as a catalyst, and also to remove the water molecules as they are formed. Click here for a list of alcohols used in this lab, and their formulas. Click here for a list of acids used in the lab, and their formulas.

If methyl alcohol is combined with acetic acid, the result is called methyl acetate.  This is a systematic way of naming any ester from the compounds from which it was made.

In today’s activity you will be preparing small samples of several different esters.  For each combination you will record the appearance, odor, and name.

Materials

acids as listed in the data table	alcohols as listed in the data table
small test tubes	250 mL beaker
400 mL beaker	hot plate or Bunsen burner
ring stand & wire gauze	concentrated sulfuric acid
baking soda (for acid spills)

Safety

N	The concentrated acetic acid is “corrosive to flesh” and the concentrated sulfuric acid can cause severe skin burns.  Use caution when handling them.
N	Besides being poisons, the alcohols are flammable.  Do not heat directly over a flame – always use a water bath.
N	Methyl alcohol is absorbed through the skin.  It attacks the optic nerve and can cause blindness, and the liver metabolizes it into formaldehyde.
N	Remember to smell your products by wafting them, not smelling them directly.

Procedure

1. Prepare a hot water bath by filling a 400 mL beaker about half full with water.  Heat it to boiling, then turn off the flame or hotplate.
2. Place 10 drops of an organic acid in a dry test tube.  If the acid is a solid, use a small spatula full (about 0.08 g).  Add 10 drops of an alcohol, and 2 drops of concentrated sulfuric acid.  Put the test tube in the beaker of hot water and let it stand for about five minutes.
3. It is dangerous to smell an ester while it is still hot, and easier to detect it’s odor if a little water is mixed in.  Combine your product with about 100 mL water in the smaller beaker, and swirl.  Carefully waft some of the vapors towards your nose so you can smell them, and record your observations.
4. If your ester smells strongly of vinegar, add a drop of phenolphthalein indicator, then drops of sodium hydroxide until it stays slightly pink.  Waft it again!
5. Prepare all of the esters as you are instructed, plus other “experimental” ones as time permits.  Complete the observation table as you go.
6. Dispose of the waste in the labeled beaker at the front of the room.

 

Required Combinations
methyl alcohol	+ acetic acid, benzoic acid, salicylic acid, 2-aminobenzoic acid
ethyl alcohol	+ acetic acid, benzoic acid, salicylic acid
iso-amyl alcohol	+ acetic acid, benzoic acid, salicylic acid
Optional Combinations
Alcohols:	n-propanol, isopropyl alcohol, n-decyl alcohol, methyl alcohol, ethyl alcohol, iso-amyl alcohol, n-butyl alcohol, t-butyl alcohol, and sec-butyl alcohol
Acids:	salicylic acid, acetic acid, sulfanilic acid, stearic acid, fumaric acid, citric acid, benzoic acid, p-aminobenzoic acid, nicotinic acid, lauric acid
acetic acid +	n-propanol, isopropyl alcohol, n-butyl alcohol, t-butyl alcohol, and sec-butyl alcohol

Results

Alcohol	Acid	Ester	Description of Odor
methyl	acetic	methyl acetate	______________________________
methyl	benzoic	_______________	______________________________
methyl	salicylic	_______________	______________________________
methyl	2-aminobenzoic	_______________	______________________________
ethyl	acetic	_______________	______________________________
ethyl	benzoic	_______________	______________________________
ethyl	salicylic	_______________	______________________________
iso-amyl	acetic	_______________	______________________________
iso-amyl	benzoic	_______________	______________________________
iso-amyl	salicylic	_______________	______________________________
_______________	_______________	_______________	______________________________
_______________	_______________	_______________	______________________________
_______________	_______________	_______________	______________________________
_______________	_______________	_______________	______________________________
_______________	_______________	_______________	______________________________

Follow Up Question
Can you think of any products in your house that might contain esters?  List a few of them, and check their ingredients.  Are any esters listed?  [You must list at least four products that you think contain esters, but you get bonus points for any ingredient lists that you bring in listing esters.]

[MHS Chem page]