unit 4 organic chemistry reactions

7/30/2019 Unit 4 Organic Chemistry Reactions

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Unit 4 Organic Chemistry Reactions

Aldehydes and Ketones

Preparation

Oxidation of a primary alcohol, usually using potassium dichromate, distilled out before it can beoxidized to a carboxylic acid.

The orange potassium dichromate will turn blue-green as chromium (III) ions are formed. []

Addit io n- El im in at io n

Anything with a H2N group, such as 2,4-DNP, 2,4-dinitrophenylhydrazine. The lone pair of electrons on the nitrogen acts as a nucleophile and forms a C=N bond with the

+ carbon in the C=O group. A water molecule is eliminated, hence addition-elimination.

With 2,4-DNP, a yellow precipitate is formed when added to a simple aldehyde.

Nucleophilic Addition

The + carbon is attacked by nucleophiles. The methyl group is electron releasing, so the longerthe carbon chain of a carbonyl compound, the more + the carbon is, so the less reactive it is.

Reaction with HCN at pH8, a base is added to the HCN as it is a very weak acid, and producesvery few CN- ions. The base is added to react the H+ ions with the OH- ions to get water, so

shifting the position of equilibrium of the weak acid.

()

This happens by nucleophilic addition.Idodoform Reaction


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Ethanal, and methlyl ketones undergo this reaction, forming a pale yellow precipitate ofiodoform.

Iodine is added to alkali solution, such as sodium hydroxide, to form iodate ions:

These substitute into the CH3 group next to the C=O bond, forming a CI3C=O group. The electronwithdrawing effect of the three halogen atoms and the oxygen atom weaken the -bond, and

this breaks, forming iodoform, CHI3.

Where R is a hydrogen atom, or an organic group.Reduction

LiAlH4 in dry ether. The H- is added to the + carbon. This is carried out in ether solution because the reagents must

be kept dry.

Add aqueous acid, to protonate the O- formed in the first step. Reduction of ethanol to ethanol:

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Reduction of propanone to propan-2-ol [] ()

Oxidation

Aldehydes can be oxidized, but not ketones. If the reaction takes place in acid or neutral solution, the product is a carboxylic acid. E.g.

potassium manganate or potassium dichromate.

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If the reaction is carried out in alkaline solution, the product is the carboxylate anion. E.g.potassium manganate, Fehlings solution, Benedicts solution or Tollens reagent.

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Potassium manganate reduces from purple to colourless in acidic solution, and from purple tobrown in neutral or alkali solution.

Potassium dichromate reduces from orange to green-blue.


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Fehlings solution reduces from deep-blue solution to a red precipitate. Benedicts solution doesthe same.

Tollens reagent is reduced from colourless to form a silver mirror.

Carboxylic AcidsPreparation

Oxidation of a primary alcohol or aldehyde usually using potassium dichromate, under reflux. The orange potassium dichromate will turn blue-green as chromium (III) ions are formed.

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Hydrolysis of an ester, heat an ester under reflux with an aqueous acid or alkali, which formseither the carboxylic acid, or its salt. Add strong acid to the salt to get the carboxylic acid.

Hydrolysis of a nitrile, heated under reflux with dilute acid, the nitrile is hydrolysed to acarboxylic acid and an ammonium salt.

Hydrolysis of a nitrile, heated under reflux with aqueous alkali forms ammonia and the salt ofthe carboxylic acid, and with the addition of a strong acid, the carboxylic acid is formed.

Iodoform reaction of a methyl ketone or secondary alcohol forms the salt of a carboxylic acid,which can be protonated to form the carboxylic acid upon the addition of strong acid.

With water

Carboxylic acids are weak acids, so form a reversible reaction with water.

With bases

Form salts with bases, such as sodium hydroxide.

With carbonates and hydrogencarbonates

React with carbonates and hydrogen carbonates to form the salt of the carboxylic acid, carbondioxide and water.

Reduction


be kept dry. The H- ion in the AlH4-group is a powerful nucleophile.

Add aqueous acid, to protonate the O- formed in the first step. Reduction of ethanoic acid to ethanol:


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Esterification

React with alchohols in the presence of concentrated sulphuric acid, as a catalyst. The catalystprotonates the ethanoic acid, which then loses a water molecule. The lone pair on the oxygen of

the alcohol bonds to the positive carbon atom on the ethanoic acid, and then a H+ is removed,

reforming the catalyst

Acid + alcohol ester + water

Formation of acid chloride

Add PCl5 or SOCl2. Clouds of steamy fumes of HCl are given off.

Formation of a halogenoacid

Bubble chlorine through boiling acid in sunlight, or UV light. A chlorine replaces one of thehydrogen atoms in the alkyl chain by free radical substitution.

Esters

Preparation

React with alchohols in the presence of concentrated sulphuric acid, as a catalyst. The catalystprotonates the ethanoic acid, which then loses a water molecule. The lone pair on the oxygen of

the alcohol bonds to the positive carbon atom on the ethanoic acid, and then a H+ is removed,

reforming the catalyst

Acid + alcohol ester + water

Hydrolysis

Hydrolysis of an ester, heat an ester under reflux with an aqueous acid to form an acid and analcohol.

Heat under reflux with aqueous alkali, which forms either the alcohol and the carboxylic acidsalt. Add strong acid to the salt to get the carboxylic acid.


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Soaps are produces by the hydrolysis of natural esters, such as vegetable oils and animal fats.Transesterification

To replace the acid with a different acid. This type of reaction is used in the manufacture of low-fat margarine, where the incoming acid is saturated, and the product acid is unsaturated.

To replace the alcohol with a different alcohol. This type of reaction is used in the manufactureof biodiesel, where the reactant alcohol is methanol or ethanol, and the product alcohol is

propane-1,2,3-triol.

Acid Chlorides

Preparation

Add PCl5 or SOCl2 to a carboxylic acid.. Clouds of steamy fumes of HCl are given off.

Water

Reacts vigorously with water to form ethanoic acid and clouds of HCl gas.

Nucleophilic substitution.Alcohols

To form an ester and misty fumes of HCl

Amm on ia

The nitrogen atom in ammonia has a lone pair, used for nucleophilic attack on the C=O group.To form an amide.


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Ami ne s

Amines also have a lone pair on the nitrogen used for nucleophilic attack to form a substitutedamide.

Reduction


be kept dry. The H- ion in the AlH4-group is a powerful nucleophile.

Add aqueous acid, to protonate the O- formed in the first step. []

Aro ma tic Co mp ou nd s

Reacts with benzene and phenol.

unit 4 organic chemistry reactions

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