Carbonyl Compounds City and Islington College

Topic Aims Level Method Equipment Duration Chemistry - Organic To help introduce students to naming and drawing Carbonyl Compounds Aimed at 16-19 year olds Level Level 3 Method In-depth PowerPoint slides, ALL can be hand-outs, some slides have questions and answers (could be done as a group discussion) Equipment Projector Laptop Pens Hand-Outs Duration >30 Mins

Learning Outcome 4th functional group – aldehydes and ketones Drawing aldehydes and ketones Naming aldehydes and ketones Physical properties of aldehydes and ketones Chemical properties of aldehydes and ketones

Carbonyl Group in Aldehydes & Ketones A carbonyl group consists of a carbon-oxygen polar double bond has a very electronegative oxygen atom has two lone pair electrons on the O atom has partial positive charge on C and partial negative charge on O

Carbonyl Group in Aldehydes & Ketones A carbonyl group in an aldehyde is attached to at least one H atom in a ketone is attached to two carbon groups

Naming Aldehydes An aldehyde has an IUPAC name in which the -e in the alkane name is changed to -al has a common name for the first four aldehydes that use the following prefixes: 1 carbon, form 2 carbons, acet 3 carbons, propion 4 carbons butyr followed by aldehyde

Naming Aldehydes

Naming Aldehydes

Naming Aldehydes Give the IUPAC name for the following aldehydes: O || CH3CH2CH2CHCH | CH3 2 methyl pentanal 3, 4 dichloro benzaldehyde

Naming Ketones When naming the following ketones in the IUPAC system, the -e in the alkane name is replaced with -one with a common name, the alkyl groups attached to the carbonyl group are named alphabetically, followed by ketone Propanone (dimethyl ketone) 2-butanone (ethyl methyl ketone)

Naming Ketones

Naming Ketones O || CH3CH2CHCH2CCH3 | CH3 Name the following ketones using the IUPAC system. O || CH3CH2CHCH2CCH3 | CH3 4 methyl 2 hexanone 2,3-dimethylcylcopentanone

Learning Check Classify each as an aldehyde or ketone. O O || || A. CH3—CH2—C—CH3 B. CH3—C—H CH3 O | || C. CH3—C—CH2—C—H D. | CH3 ketone aldehyde aldehyde ketone

Learning Check Classify each as an aldehyde, ketone, alcohol, or ether. O A. CH3CH2CCH3 B. CH3OCH3 CH3 O OH C. CH3CCH2CH D. CH3CHCH3 CH3 ketone ether alcohol aldehyde

Learning Check Give the IUPAC name for each of the following ketones. O A. CH3CH2CH2CH2CCH3 B. ClCH2CH2CCH3 2-hexanone 4-chloro-2-butanone

Learning Check Give the IUPAC name for the following aldehyde compounds. O || A. CH3—CH2—CH2—C—H CH3 O | || B. CH3—C—CH2—C—H | CH3 butanal 3,3-dimethylbutanal

Learning Check Draw the structural formulas for each: 4-methylpentanal 2,3-dichloropropanal 3-methyl-2-butanone

Bonding & Reactivity Oxygen is far more electronegative than carbon and so has a strong tendency to pull electrons in a carbon-oxygen bond towards itself. One of the two pairs of electrons that make up a carbon-oxygen double bond is even more easily pulled towards the oxygen. That makes the carbon-oxygen double bond very highly polar.

Continue.... The slightly positive carbon atom in the carbonyl group can be attacked by nucleophiles. A nucleophile is a negatively charged ion (for example, a cyanide ion, CN-), or a slightly negatively charged part of a molecule (for example, the lone pair on a nitrogen atom in ammonia, NH3). During the reaction, the carbon-oxygen double bond gets broken. The net effect of all this is that the carbonyl group undergoes addition reactions, often followed by the loss of a water molecule. This gives a reaction known as addition-elimination or condensation. Both aldehydes and ketones contain a carbonyl group. That means that their reactions are very similar in this respect.

Where Aldehydes & Ketones differ? An aldehyde differs from a ketone by having a hydrogen atom attached to the carbonyl group. This makes the aldehydes very easy to oxidise. For example, ethanal, CH3CHO, is very easily oxidised to either ethanoic acid, CH3COOH, or ethanoate ions, CH3COO-. Ketones don't have that hydrogen atom and are resistant to oxidation. They are only oxidised by powerful oxidising agents which have the ability to break carbon-carbon bonds.

Physical Properties of Aldehydes & Ketones The polar carbonyl group provides dipole-dipole interactions. + - + - C=O C=O Because the electronegative oxygen atom forms hydrogen bonds with water molecules, aldehydes and ketones with one to four carbons are very soluble.

Solubility of Some Aldehydes & Ketones

Solubility in Water The electronegative O atom of the carbonyl group of aldehydes and ketones forms hydrogen bonds with water.

Addition to Aldehydes & Ketones Addition of hydrogen cyanide to aldehydes and ketones ethanal (an aldehyde) propanone (a ketone) Hydrogen cyanide adds across the carbon-oxygen double bond in aldehydes and ketones to produce compounds known as hydroxynitriles. These used to be known as cyanohydrins

Reduction of Aldehydes & Ketones Aldehydes and ketones can be reduced by sodium borohydride, NaBH4, or H2. The reduction of aldehydes and ketones decreases the number of carbon–oxygen bonds by addition of hydrogen or loss of oxygen.

Reminder! Primary alcohols can be oxidized to aldehydes, which can be easily oxidized to carboxylic acids. Secondary alcohols can be oxidized to ketones, which cannot be further oxidized.

Oxidation of Aldehydes &Ketones Why do aldehydes and ketones behave differently? You will remember that the difference between an aldehyde and a ketone is the presence of a hydrogen atom attached to the carbon oxygen double bond in the aldehyde. Ketones don't have that hydrogen.

Continue.... The presence of that hydrogen atom makes aldehydes very easy to oxidise. Or, put another way, they are strong reducing agents. Because ketones don't have that particular hydrogen atom, they are resistant to oxidation. Only very strong oxidising agents like potassium manganate(VII) solution (potassium permanganate solution) oxidise ketones - and they do it in a destructive way, breaking carbon-carbon bonds. Provided you avoid using these powerful oxidising agents, you can easily tell the difference between an aldehyde and a ketone. Aldehydes are easily oxidised by all sorts of different oxidising agents: ketones aren't.

What is formed when Aldehydes are Oxidised?

Acidified potassium dichromate(VI) solution The orange dichromate(VI) ions have been reduced to green chromium(III) ions by the aldehyde. In turn the aldehyde is oxidised to the corresponding carboxylic acid.

Tollens’ Test (the silver mirror test) In Tollens’ test, Tollens’ reagent, which contains Ag+, oxidizes aldehydes but not ketones. Ag+ is reduced to metallic Ag, which appears as a “mirror” in the test tube.

Tollens’ Test (the silver mirror test)

Benedict’s Test In Benedict’s test, Benedict’s reagent, which contains Cu2+, reacts with aldehydes that have an adjacent OH group.

Benedict’s Test In Benedict’s test, an aldehyde is oxidized to a carboxylic acid, while Cu2+ is reduced to give red Cu2O(s). The blue Cu2+ in Benedict’s solution forms a brick-red solid of Cu2O in a positive test for many sugars and aldehydes with adjacent hydroxyl groups.

Addition-elimination reactions of Aldehydes & Ketones The reaction with 2,4-dinitrophenylhydrazine 2,4-dinitrophenylhydrazine is often abbreviated to 2,4-DNP or 2,4-DNPH. a solution of 2,4-dinitrophenylhydrazine in a mixture of methanol and sulphuric acid is known as Brady's reagent.

The Chemistry of the Reaction aldehyde or ketone Brady's reagent A bright orange or yellow precipitate shows the presence of the carbon-oxygen double bond in an aldehyde or ketone.

Continue..... The reaction is known as a condensation reaction. A condensation reaction is one in which two molecules join together with the loss of a small molecule in the process. In this case, that small molecule is water. In terms of mechanisms, this is a nucleophilic addition-elimination reaction. The 2,4-dinitrophenylhydrazine first adds across the carbon-oxygen double bond (the addition stage) to give an intermediate compound which then loses a molecule of water (the elimination stage).

Using the Reaction The reaction has two uses in testing for aldehydes and ketones. First, you can just use it to test for the presence of the carbon-oxygen double bond. You only get an orange or yellow precipitate from a carbon-oxygen double bond in an aldehyde or ketone. Secondly, you can use it to help to identify the specific aldehyde or ketone. The precipitate is filtered and washed with, for example, methanol and then recrystallised from a suitable solvent which will vary depending on the nature of the aldehyde or ketone. For example, you can recrystallise the products from the small aldehydes and ketones from a mixture of ethanol and water. The crystals are dissolved in the minimum quantity of hot solvent. When the solution cools, the crystals are re-precipitated and can be filtered, washed with a small amount of solvent and dried. They should then be pure. If you then find the melting point of the crystals, you can compare it with tables of the melting points of 2,4-dinitrophenylhydrazones of all the common aldehydes and ketones to find out which one you are likely to have got.

Some other similar Reactions 1. The product is a "hydrazone". If you started from propanone, it would be propanone hydrazone. 2. The product is an "oxime" - for example, ethanal oxime.

Concept Map Oxidation reactions Aldehydes & Ketones Addition of hydrogen cyanide Addition – Elimination reactions (2,4 – DNP) Reduction of Aldehydes & Ketones Oxidation reactions

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