CM2201 L4 - lecture notes PDF

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CM2201: L4: Organic Chemistry: the carbonyl functional group:

Very important functional group ● Reactivity and properties depend strongly on the substituents ● Must identify which derivative is present ● Term “carbonyl group” is too general

Nomenclature of Aldehydes (ending in -al):

● ● ● ● ●

The parent cain contains C=) (CHO) Use the usual greek stem to indicate the number of carbons in the parent chain Add the suffix “-al” to the stem CHO must be terminal → no need to number it Indicate the position of substituents by usual method of numbering, relative to position of CHO

Nomenclature of ketones (-one):

● ● ● ●

The parent chain contains C=O Use the usual greek stem to indicate the number of carbons in the parent chain Add the suffix “-one” to the stem Indicate the position of the C=O by the usual method of numbering - number so as to give it lowest number possible ● Indicate position of substituents by usual method of numbering, relative to the position of the C=O Aldehydes and ketones in food:

Structure of the carbonyl group:

Nucleophilic addition to the carbonyl group: ● Electrophilic carbon is susceptible to attack by nucleophiles (nucleophilic attack)

Nucleophilic addition of alcohols - hemiacetals: ● Nucleophilic addition of alcohol to aldehydes - formation of hemiacetals and acetals

● Alcohols : O is the nucleophilic atom

● Nucleophilic addition of alcohol to aldehydes - formation of hemiacetals and acetals ● Alcohols: O is the nucleophilic atom

Nucleophilic addition of alcohols - acetals: ● In the presence of excess alcohol , hemiacetals react further, giving acetals

Mechanism of conversion of hemiacetal to acetal:

Aldehydes, hemiacetals and acetals in carbohydrates:

Nucleophilic addition of amines - imine formation: ● Nucleophilic addition of primary amines to aldehydes or ketones - formation of imines ● Amines: N is the nucleophilic atom (has a lone pair of electrons)

Imines in foods:

Oxidation aldehydes:

● Difference in reactivity towards oxidation is used as a test to distinguish aldehydes and ketones ● Tollen’s Silver mirror test

Reduction of aldehydes and ketones:

● 2 common reducing agents used are: ○ LiAlH4 (lithium aluminium hydride) ○ NaBH4 (sodium borohydride) ● LiAlH4 is more reactive - it is a stronger reducing agent than NaBH4 Biological reduction and oxidation of C=O:

Carboxylic acids: Nomenclature:

Physical properties of carboxylic acids: ● Short chain derivatives are liquid with sharp odours ○ Eg acetic acid (vinegar 4-5%)butanoic acid (rancid butter) ● Polar compounds: ○ Form hydrogen bonds ○ High boiling points for their molecular weights ○ Boiling points are even higher than those of alcohols

Acidity of carboxylic acids: ● RCOOH - relatively weak acids ● They dissociate in water to give carboxylate ion (anionic) and hydronium ion ○ I.e. They are proton donors

● The above reaction (dissociation) is reversible (hence the equilibrium arrows) ● The equilibrium constant K for this reaction is expressed as follows:

Acid dissociation constant, Ka:

● H2O is the solvent, so it is present in massive excess over other entities ---> [H2O] is essentially constant ● We express a modified constant, Ka, called the “acid dissociation constant”

● The more dissociation of and acid into carboxylate ion + H 3O+, the larger the Ka is → Ka gives a measure of the strength of the acid

pKa:

● We define pne fourther quantity(used as ameasure of acid strength)

● The lower the pKa value the stronger and acid is pKa of RCOOH vs. ROH:

Resonance stabilisation of the Carboxylate Ion:

● Negative charge equally shared (delocalised) over the 2 oxygen atoms (as represented in the resonance hybrid structure) ● This stabilises the carboxylate anion considerably ● No resonance stabilisation in ethoxide anion CH3CH2O- (negative charge is localised on one oxygen atom) Carboxylate salts:

Fatty acids: ● Key components of lipids ● Long alkyl chains (10 to 20 carbons): carboxylic acid (head) ● No double bonds = saturated ● Double bonds = unsaturated ● Carboxylate salts have s...