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

 GCE-AS-A2-IB ADVANCED LEVEL ORGANIC CHEMISTRY

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Structure & Naming of Alcohols & Ethers notes and quizzes

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Naming aldehydes or ketones QUIZ: multiple choice or type in name quiz

and (iii) notes on comparing alcohol and phenol structures and

  oxidation sequence: alcohol ==> aldehyde/ketone ==> carboxylic acid

• 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). The functional group is shown by using 'al' in the suffix part of the name e.g. methanal, ethanal, propanal etc.
• 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). The functional group is shown by using 'one' in the suffix part of the name e.g. propanone, butanone, hexan-3-one etc.
• The substituent numbers are based on giving the 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.

1. methanal (formaldehyde), , ,

2. ethanal (acetaldehyde), , , ,

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), , , ,

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

   • , , ,

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

Examples of iodoketones

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)

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).

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.

Phenols ROH, R=aryl (i.e. the -OH group is directly attached to a benzene ring): Some are quite stable to oxidation, but others, like phenol itself, are quite readily oxidised to complex aromatic products. Examples of phenols …

1.  , , , phenol (for more Aromatic compounds)

2. 2-chlorophenol  (the phenol group is given position '1')

3. 3-methylphenol, and 2,5-dichloro-4-methylphenol

4.  , 2-hydroxybenzoic acid or 2-hydroxybenzenecarboxylic acid (it is both a phenol and a carboxylic acid BUT the -COOH group is 'higher ranking' so the phenol group is quoted as a substituent).

GENERAL REVISION NOTES

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