Download
1 / 38
570 likes | 1.11k Vues
The carbonyl group. The carbonyl group (>C=O) is the functional group found in compounds such as aldehydes , ketones , and carboxylic acids. In aldehydes the carbonyl group is at the end of the carbon chain and so has at least one hydrogen attached to it.
E N D
The carbonyl group The carbonyl group (>C=O) is the functional group found in compounds such as aldehydes, ketones, and carboxylic acids. • In aldehydes the carbonyl group is at the end of the carbon chain and so has at least one hydrogen attached to it. • In ketones the carbonyl group is in the middle of a carbon chain and so has two alkyl groups attached to it.
Representing aldehydes and ketones Aldehydes and ketones cannot be distinguished by their molecular formula because they have the same functional group. What is the molecular formula of the aldehyde and ketone below? an aldehyde a ketone Both have the molecular formula C3H6O
Representing aldehydes and ketones Aldehydes and ketones are therefore represented either using displayed or structural formulae. CH3CHO CH3CH2CH2CHO CH3COCH2CH3 CH3CH2COCH2CH3
Naming aldehydes Aldehydes are named using the suffix –al, and follow the same conventions as for naming alkanes. ethanal propanal 2-methylbutanal
Naming ketones Ketones are named using the suffix –one. hexan-3-one propanone Like alkenes, ketones with four or more carbon atoms display positional isomerism because the carbonyl group may appear between different carbon atoms. In these cases, a number is used before the –one to indicate the first carbon involved.
Properties of aldehydes and ketones The carbonyl group is polar due to the greater electronegativity of oxygen (3.4) than carbon (2.6). This influences the properties of aldehydes and ketones, such as solubility. δ+ δ- Small aldehydes and ketones are soluble in water due to hydrogen bonding between a lone pair on the oxygen of the carbonyl group and the hydrogen of the water. As size increases, solubility decreases due to interference in hydrogen bonding by the hydrocarbon ‘tails’ of the aldehydes/ketones.
Boiling points and intermolecular forces The general increase in boiling points from alkanes to aldehydes/ketones and alcohols is due to the intermolecular forces between each type of molecule. • Alkanes are only held together by van der Waals forces. These forces increase with the size/length of a molecule. • The polar carbonyl group in aldehydes and ketones means that as well as van der Waals forces, these molecules are also held together by dipole–dipole interactions. • Alcohols are held together by van der Waals forces and dipole–dipole interactions. In addition, these molecules can form hydrogen bonds with each other, due to the slightly positive hydrogen atom of the hydroxyl group.
Reactivity of the carbonyl group Although double bonds require more energy to break than single bonds, compounds with double bonds tend to be more reactive as addition reactions are possible. Some of the chemical properties of aldehydes and ketones result from the polar nature of the C=O bond. δ+ δ- • The positive charge on the carbon atom makes it open to attack by nucleophiles. • Aldehydes and ketones can be reduced, forming primary and secondary alcohols respectively. • Aldehydes may also be oxidized to carboxylic acids.
Dangers of HCN HCN(aq)H+(aq) + CN-(aq) Hydrogen cyanide (HCN) is highly volatile liquid (boiling point 26°C), which has a faint bitter almond smell. In solution, hydrogen cyanide partially dissociates: Hydrogen cyanide is highly toxic because it inhibits a mitochondrial enzyme that is essential for respiration. Being so volatile and flammable, it is difficult to handle safely. A safer alternative is potassium cyanide, which is a solid at room temperature and is therefore easier to handle. An acidified solution contains both the H+ and CN- ions.
Carboxylic acids Carboxylic acids have a carboxyl group(-COOH) consisting of a carbonyl group and a hydroxyl group attached to the terminal carbonyl carbon. Carboxylic acids are named using the suffix –oic acid. Methanoic acid is the simplest carboxylic acid and is found in bee and ant stings. Ethanoic acid is the acid that gives vinegar its sharp taste and smell. It is also important in the chemical industry and about 6.5 million tonnes are used worldwide each year
Oxidation of 1° alcohols and aldehydes Carboxylic acids can be created by the oxidation of aldehydes by oxidizing agents such as potassium dichromate(VI). They can also be created by the oxidation of primary alcohols, again using potassium dichromate(VI).
Hydrolysis of nitriles Carboxylic acids can also be prepared by the hydrolysis of nitriles. The nitrile is refluxed with water and hydrochloric acid. The carboxylic acid can then be distilled from the reaction mixture.
Reactivity of carboxylic acids δ- The reactivity of carboxylic acids results, in part, from the polarization of its bonds. δ+ δ- δ+ The reactions of carboxylic acids include: • neutralisation – the carboxylic acid loses a proton to form a carboxylate salt • nucleophilic substitution – the positively-charged carbon is attacked by a nucleophile, resulting in substitution of the OH group • esterification – reaction with an alcohol to form an ester.
Forming the carboxylate ion Carboxylic acids are weak acids and partially dissociate in aqueous solution: carboxylate ion The negative charge is delocalized across the carboxylate group, resulting in a more stable ion. The delocalization is represented by adding a second dotted line to the carbon oxygen bonds, which are both equivalent.
Reactions with carbonates 2CH3COOH + Na2CO3→ 2CH3COONa + CO2 + H2O HCOOH + NaHCO3→ HCOONa + CO2 + H2O Carboxylic acids are the only organic compounds that are strong enough acids to react with either sodium carbonate or sodium hydrogencarbonate. Reaction of ethanoic acid and sodium carbonate: sodium ethanoate Reaction of methanoic acid and sodium hydrogencarbonate: sodium methanoate The formation of bubbles of carbon dioxide gas makes these reactions useful as a test for carboxylic acids.
Reactions with alkalis RCOOH + NaOH → RCOONa + H2O CH3CH2COOH + NaOH → CH3CH2COONa + H2O CH3CH2CH2COOH + KOH → CH3CH2CH2COOK + H2O Carboxylic acids will react with alkalis, such as sodium hydroxide, in a neutralization reaction: Reaction of propanoic acid and sodium hydroxide: sodium propanoate Reaction of butanoic acid and potassium hydroxide: potassium butanoate
