Carbohydrates: Quick Revision

A brief note on Carbohydrates. Read it before a test. Try each question at the end before you open its answer.

Classification

• A carbohydrate is a polyhydroxy aldehyde or ketone, or gives one on hydrolysis.
• Monosaccharides (glucose, fructose), oligosaccharides (2 to 10 units, such as sucrose) and polysaccharides (starch, cellulose, glycogen).
• Aldose: C=O at C-1. Ketose: C=O at C-2. Glucose is an aldohexose and fructose a ketohexose.
• Reducing sugars reduce Benedict's, Fehling's and Tollens' reagents. They have a free anomeric carbon.

Stereochemistry

• $n$ stereocentres give $2^n$ stereoisomers. An aldohexose has 4 centres and 16 isomers. A ketohexose has 3 and 8.
• D or L depends on the OH on the last stereocentre (right is D). It is not the sign of the rotation.
• Epimers differ at one stereocentre. Mannose is the C-2 epimer of glucose. Galactose is the C-4 epimer.

Glucose

• Evidence: $\text{C}_{6}\text{H}_{12}\text{O}_{6}$, a straight chain (n-hexane with HI), five OH (pentaacetate), an aldehyde (bromine water gives gluconic acid) and a primary OH (nitric acid gives saccharic acid).
• The C-5 OH closes the ring: a six-membered pyranose with a hemiacetal at C-1.
• Anomers: α has the C-1 OH down and β has it up (D sugars, Haworth ring). Right in Fischer is down in Haworth.
• Mutarotation: $+112.2$ (α) and $+18.7$ (β) degrees go to $+52.7$ degrees, about 36% α and 64% β.
• Glucose and fructose both give Tollens', Fehling's and the same osazone.

Disaccharides and polysaccharides

• Sucrose: α(1→2)β, non-reducing, gives glucose and fructose (invert sugar, $+66.5$ to $-19.9$ degrees).
• Maltose: α(1→4), reducing. Lactose: β(1→4), reducing, gives galactose and glucose.
• Starch: amylose (about 20%, unbranched, α(1→4)) and amylopectin (about 80%, α(1→4) with α(1→6) branches).
• Cellulose: β(1→4), straight fibres, not digested by humans. Glycogen: highly branched animal store.
• $n$ units joined in a chain release $n - 1$ molecules of water.

A worked example

Find the molar mass of a chain of 100 glucose units.

1. Links. One hundred units have 99 links.
2. Mass. $100 \times 180.16 - 99 \times 18.02 = 18016 - 1784 = 16232\ \text{g/mol}$.

Practice with new numbers

A program to try

Mistakes to avoid

• Counting units as links. A chain of $n$ units has $n - 1$ links.
• Calling sucrose reducing. Both anomeric carbons are in the link.
• Mixing D with the sign (+). They are different ideas.
• Calling mannose an anomer of glucose. It is an epimer.

Check yourself

1. How many stereoisomers has an aldopentose?

2. Which anomer of D-glucopyranose has the C-1 OH on the same face as the CH2OH group?

3. Why does sucrose not reduce Fehling's solution?

4. How many molecules of water are released when 30 glucose units join in a chain?

Sources

• Nelson and Cox, Lehninger Principles of Biochemistry, 8th edition, Chapter 7.
• NCERT Chemistry for Class 12, Part II, the chapter Biomolecules.
• OpenStax, Organic Chemistry, Chapter 25.