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Chemistry122Updated 5 Oct 20263 pages

Amino Acids, Amides, and Chirality - OCR Organic Chemistry Module 6

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Ever wondered how the proteins in your muscles are built, or why some medicines work differently on different people? It all comes down to amino acids and their fascinating chemical properties, including their ability to exist as mirror images of each other.
Chapter 27.2 - Amino acids, Amides and Chirality – page 1

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Amino Acids and Their Chemical Behaviour

Amino acids are the building blocks of proteins, and they're pretty clever molecules because they contain both an amine group (NH₂) and a carboxyl group (COOH). This dual personality makes them incredibly versatile in chemical reactions.

Think of amino acids as chemical chameleons. The COOH group can act as an acid (donating protons), whilst the NH₂ group can act as a base (accepting protons). This means amino acids can react with both acids and bases to form salts.

For example, when 2-aminoethanoic acid reacts with sodium hydroxide (NaOH), the carboxyl group loses a proton. When it reacts with hydrochloric acid (HCl), the amine group gains a proton. This ability to react with both acids and bases is what makes amino acids so important in biological systems.

Quick Tip: Remember that amino acids are amphoteric - they can behave as both acids and bases depending on the conditions!

Chapter 27.2 - Amino acids, Amides and Chirality – page 2

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Esterification and Types of Isomers

The carboxyl group in amino acids can also undergo esterification when reacted with alcohols under acidic conditions. This reaction produces an ester and water, just like other carboxylic acid reactions you've seen before.

Now, let's tackle isomers - molecules that share the same molecular formula but differ in other ways. Structural isomers have different structural formulas entirely, like completely different arrangements of atoms.

Stereoisomers are more subtle - they have the same molecular and structural formulas but different spatial arrangements. This includes E/Z isomers, which exist around C=C double bonds where rotation is restricted, and optical isomers, which are non-superimposable mirror images.

Remember: For E/Z isomers to exist, each carbon in the C=C bond must be connected to two different substituents - no rotation allowed!

Chapter 27.2 - Amino acids, Amides and Chirality – page 3

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Optical Isomerism and Chirality

Optical isomers are like your left and right hands - they're mirror images that can't be superimposed on each other, no matter how you twist them. This happens when you have a chiral carbon atom bonded to four different groups.

The chiral carbon adopts a tetrahedral shape with bond angles of 109.5°. Each chiral carbon produces exactly two optical isomers, called a pair of enantiomers. These molecules are identical in most properties but rotate light in opposite directions.

It's crucial to remember that carbons involved in double bonds or benzene rings can't be chiral because they don't have four different substituents. Many amino acids contain chiral carbons, which is why this concept is so important in biochemistry.

Key Point: Look for carbons bonded to four different groups - that's your chiral centre and the source of optical isomerism!

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