Doc Brown's GCE A Level AS A2 Chemistry  Revising Advanced Level Organic Chemistry

Revision Notes Part 5 CARBONYL COMPOUNDS - ALDEHYDES and KETONES

5.1 The molecular structure and naming of ALDEHYDES and KETONES - including nomenclature of some isomers

How to name aldehydes? How to name ketones? Nomenclature of substituted aldehydes or ketones - examples of acceptable names, displayed formula of aldehydes and ketones, graphic formula, molecular formula, skeletal formula, structural formula of these homologous series and how to name the carbonyl group of compounds known as aldehydes and ketones

Organic Chemistry Part 5 sub-index: 5.1.1 Nomenclature introduction * 5.1.2 Examples of aldehydes * 5.1.3 Examples of ketones * 5.1.4 Other examples of substituted ketones * 5.1.5 Oxidation sequence: alcohol ==> aldehyde/ketone ==> carboxylic acid * Other advanced level organic chemistry links

5.1.1 Introduction to Aldehyde and Ketone Nomenclature

• How do you name aldehydes? How do you name ketones? How do you name substituted aldehydes or ketones?
• Aldehydes and ketones are a group of compounds containing the carbonyl group, C=O.
• Aldehydes always have a hydrogen atom attached to the carbon of the carbonyl group, so the functional group is -CHO (see diagram above).
  • The functional group is shown by using 'al' in the suffix part of the name e.g. methanal, ethanal, propanal etc.
  • The prefix for the aldehyde name is based on the parent alkane minus the e.
  • No number is required for the aldehyde group because the aldehyde group cannot be anything else except carbon atom 1.
    •  A number would only be required if a higher ranking group is present e.g. a -COOH carboxylic acid group.
• Ketones always have two carbon atoms attached to the carbon atom of the carbonyl group, so the functional group is C-CO-C (see diagram above).
  • The prefix for the ketone name is based on the parent alkane minus the e.
  • The functional group is shown by using 'one' in the suffix part of the name e.g. propanone, butanone, hexan-3-one etc.
  • A number to denote the position of the ketone group is definitely required beyond butanone and although not strictly needed for butan-2-one, since only one ketone position is possible, it is required for substituted butanones and beyond.
  • The substituent numbers are based on giving the carbonyl C=O carbon the lowest number e.g. 2-methylbutanal ('al' position = 1). The number position of the C=O group in ketones needs to be specified for carbon chains of over 4, or less, if substituents present e.g. 3-methylbutan-2-one, heptan-2-one, heptan-3-one and heptan-4-one (there is no heptan-1-one, this is heptanal!).
• For the same 'carbon number', aldehydes and ketones are structural and functional group isomers based on e.g. for aliphatic carbonyl compounds C_nH_2nO.
• Some 'old' names are quoted in (italics) though their use should be avoided if possible [but many still used - just put one into GOGGLE!].

1. methanal (formaldehyde), , ,

Methanal is a trigonal planar shape and the planarity gives H-C=O and H-C-H bond angles of 120^o.

2. ethanal (acetaldehyde), , , ,

Bond angles from left to right: H-C-C 109^o, C-C=O 120^o, C-C-H 120^o and O=C-H 120^o.

3. propanal (propionaldehyde), , , ,

4. 2-methylpropanal (2 not strictly needed but advisable, 2-methypropionaldehyde),

   • , ,

5. butanal (butyraldehyde), , ,

6. 2-methylbutanal (2-methylbutyraldehyde), ,

7. 3-methylbutanal (3-methylbutyraldehyde), ,

8. pentanal (old name 'valeraldehyde'), ,

9. benzaldehyde (benzenecarbaldehyde), and 2-hydroxybenzaldehyde

1. propanone (acetone, dimethyl ketone, DMK), , , ,

Bond angles: The trigonal planarity around the carbonyl group >C=O gives a C-C=O and C-C(=O)-C of 120^o and associated with the methyl group, the H-C-C and H-C-H bond angles are 109^o.

2. butanone (or butan-2-one, but 2 not strictly needed, methyl ethyl ketone, MEK, 2-butanone),

   • , , ,

   • Bond angles: H-C-C, H-C-H and (O=)C-C-C on right are all 109^o and C-C=O on left, C-C(=O)-C and O=C-C on right are 120^o and the angles associated with the methyl group on the right are all 109^o.

3. 3-methylbutan-2-one (2 and 3 not strictly needed BUT advisable, 3-methyl-2-butanone),

   • ,

4. pentan-2-one (2-pentanone), , ,

5. pentan-3-one (3-pentanone), , ,

6. 1-phenylethanone

7. 2-hydroxyphenylethanone

5.1.4 Other examples of substituted ketones e.g. iodoketones and phenylketones

1 to 3, 5 to 6 and 10 are formed by the reaction of the parent ketone with iodine in the presence of an acid (e.g. HCl_(aq), because H⁺_(aq) catalyses the reaction)

1. iodopropanone (1-iodopropanone, 1- not really needed)

2. 1-iodobutanone (1-iodobutan-2-one, -2- not really needed, 1-iodo-2-butanone)

3. 3-iodobutanone (3-iodobutan-2-one, -2- not really needed, 3-iodo-2-butanone)

4. 4-iodobutanone (4-iodobutan-2-one, -2- not really needed, 4-iodo-2-butanone)

5. 1-iodopentan-2-one (-2- needed, 1-iodo-2-pentanone)

6. 3-iodopentan-2-one (-2- needed, 3-iodo-2-pentanone)

7. 4-iodopentan-2-one (-2- needed, 4-iodo-2-pentanone)

8. 5-iodopentan-2-one (-2- needed, 5-iodo-2-pentanone)

9. 1-iodopentan-3-one  (-3- needed, 1-iodo-3-pentanone)

10. 2-iodopentan-3-one (-3- needed, 2-iodo-3-pentanone)

11. Phenylketones e.g.

    • C₆H₅CH₃COCH₂CH₃  is 1-phenylbutan-2-one (1-phenyl-2-butanone)

    • CH₃COCH(C₆H₅)CH₃  is 3-phenylbutan-2-one (3-phenyl-2-butanone)

    • CH₃COCH₂CH₂C₆H₅  is 4-phenylbutan-2-one (4-phenyl-2-butanone)

12. -

Alcohols can be readily oxidised to aldehydes and ketones and aldehydes are easily oxidised further to carboxylic acids. The reagent can potassium dichromate(VI) K₂Cr₂O₇ , acidified with diluted sulphuric acid H₂SO_4(aq) (colour change is orange to green).

For details of the practical method see Organic Redox Reactions

However the oxidation products depend on the original structure of the alcohol. The alcohol functional group -OH in aliphatic alcohols is classified into primary, secondary and tertiary types (see below). When the -OH is attached directly to a benzene ring the molecule is called a phenol.

Primary aliphatic alcohols R-OH, R is H or alkyl: When oxidised they form aldehydes and then further oxidation gives a relatively stable carboxylic acid e.g.

1. ==>==>

   • ethanol ==> ethanal ==> ethanoic acid

2. ==>==>

   • 2-methylpropan-1-ol ==> 2-methylpropanal ==> 2-methylpropanoic acid

3.  ==> ==>

   • butan-1-ol ==> butanal ==> butanoic acid

4. ==>==>

   • pentan-1-ol ==> pentanal ==> pentanoic acid

Secondary aliphatic alcohols R-CH(OH)-R', R or R' are both alkyl (can be aryl): When oxidised they form relatively stable ketones (see NOTE below) e.g.

1.  ==> , propan-2-ol ==> propanone

2.  ==> , butan-2-ol ==> butanone (butan-2-one)

3.  ==> , pentan-3-ol ==> pentan-3-one

Tertiary aliphatic alcohols RR'R"C-OH, where R,R' or R" are all alkyl (or aryl): These are relatively stable to oxidation (see NOTE 1. further down) e.g.

1. 2-methylpropan-2-ol , or

2. 2-methylbutan-2-o l, or

3. 3-methylpentan-3-o l, or

1. Ketones and carboxylic acids are relatively stable to further oxidation because a strong C-C bond must be broken in the process. Prolonged oxidation with H₂SO_4(aq)/K₂Cr₂O₇ or using a more powerful oxidising agent, results in the formation of carbon dioxide, water and carboxylic acids of shorter carbon chain length than the original alcohol or ketone.

2. When a primary alcohol is oxidised to an aldehyde, the oxidation to the carboxylic acid is rapid. If the aldehyde formed first is the desired product, it must be immediately distilled off to prevent further oxidation.

LINKS TO ASSOCIATED ADVANCED ORGANIC CHEMISTRY PAGES

Multiple choice quiz on naming aldehydes or ketones

Type in name (short answer) quiz on aldehydes and ketones

10.6.1 Carbonyl compounds - ALDEHYDES and KETONES - introduction to their reactions and reaction mechanism

10.6.2 Nucleophilic addition of hydrogen cyanide to aldehydes & ketones to form a hydroxy-nitrile

10.6.3 Addition of hydrogen to aldehydes and ketones - reduction with LiAlH₄ or NaBH₄ to give alcohols

10.6.4 Iodination of ketones e.g. a 2-one like propanone (a methyl ketone) to give iodo-ketones

All Advanced Organic Chemistry Notes

Summary of Organic Functional Groups Notes

Quiz on Organic Structure Recognition

Summary of organic chemistry functional group tests

Intermolecular forces & boiling points of aldehydes & ketones compared to other organic molecules

The shapes and bond angles of simple organic molecules

A Level Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry, WJEC GCE AS A2 Chemistry, CCEA/CEA GCE AS A2 Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and AP Honours/honors level courses)

formula keywords: how to name naming nomenclature empirical molecular formula graphic formula displayed formula skeletal formula structural isomers isomerism 5.1.1 carbonyl compounds Nomenclature introduction * 5.1.2 Examples of aldehydes * 5.1.3 Examples of ketones * 5.1.4 Other examples of substituted ketones  CH2O HCHO C2H4O CH3CHO CH3-CHO C3H6O CH3CH2CHO CH3-CH2-CHO CH3COCH3 CH3-CO-CH3 C4H8O CH3CH2CH2CHO CH3-CH2-CH2-CHO CH3COCH2CH3 CH3-CO-CH2-CH3 C5H10O CH3CH2CH2CH2CHO CH3COCH2CH2CH3 CH3CH2COCH2CH3 C7H6O C6H5CHO C6H5COCH3

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