Carbohydrates

Carbohydrates

These include monosaccharides, disaccharides and polysaccharides.

Carbohydrates contain Carbon, Hydrogen and Oxygen.

H : 0

2 : 1 ratio

Therefore, twice as many Hydrogen atoms as Oxygen atoms.

Monosaccharides

Have low molar mass (molecular mass) and they usually have 3, 5 or 6 carbon atoms.

TRIOSES C3H6O3 e.g. glyceraldehyde - Krebs cycle in respiration

PENTOSES C5H10O5 e.g. ribose, in DNA/RNA, photosynthesis

HEXOSES C6H12O6 e.g. glucose, fructose - respiration, photosynthesis

Properties of Monosaccharides

1. Sweet taste.

2. Small and soluble (OH groups are polar making the sugar soluble).

3. As solids they are crystalline (crystal form)

(due to polar OH groups)

4. When they are oxidised a lot of energy is released.

e.g. for glucose, energy content = 2880 kJ mole-1 (mole-1 means per mole).

Therefore get energy from sugar.

There are two functional groups in a monosaccharide.

( Functional groups = parts of the molecule that take part in reactions.)

These are:

„o OH group.

called a carbonyl group ÐŽV( books refer to this as aldehyde or keto group, depending on type of sugar)

Background knowledge:

OXIDATION / REDUCTION

Oxidation = add O; loss of e-; loss of H

e.g. Ca + O „_ CaO2 Ca loses 2 electrons to the oxygen.

Reduction = loss of O; gain of electrons; gain H

TEST FOR REDUCING SUGAR

BenedictÐŽ¦s solution is added, and the mixtures heated to 95oC. If a red precipitate (not soluble) is produced then reducing sugar is present.

The BenedictÐŽ¦s contains Cu2+ [same as Cu (II)].

The copper atom has lost 2 e-, i.e. valency = 2.

Cu2+ is soluble with a blue colour.

Reducing Sugars (All monosaccharides, some disaccharides)

- have a carbonyl group.

The carbonyl group releases electrons in the test.

Cu (I) gives off a red colour. It is insoluble and settles to the bottom.

Therefore these sugars are called reducing sugars.

Non-Reducing Sugars

e.g. sucrose

Carbonyl group or groups are not able to release electrons - locked into a chemical bond called 1,2, glycosidic bond.

Isomerism

Some molecules may exist in more than one form, according to the positions of groups within the molecule.

Two types of isomerism:

1. Structural Isomers

mer = molecule

(same/similar molecules) iso = same

There are three forms of glucose. Straight chain, Ñ"С (alpha) and Ñ"Т (beta)

In the straight chain form of glucose (notes not needed) 1C has the carbonyl group (double bond to oxygen).

In the ring form of glucose 1C is linked by oxygen to 5C.

The difference between alpha (Ñ"С) glucose & beta (Ñ"Т) glucose is that in alpha glucose the OH on 1C is below, but in beta glucose the OH is above.

Ñ"С & Ñ"Т Glucose and fructose all have the same number of carbon, hydrogens and oxygens therefore they are structural isomers.

2. Optical Isomers

Some sugars occur in two forms that differ in the way they affect polarised light.

Dextro - natural sugars ÐŽV rotate plane of polarised light to right.

Laevo - man-made - rotate plane of polarised light to left.

(Used as artificial sweeteners.)

Disaccharides

Disaccharides form by a condensation reaction (water being removed from the molecules) joining 2 monosaccharides together.

Maltose

Usually 1C on one monosaccharide linking to 4C on the other. The bond that is formed is known as a glycosidic bond