[26.1] carbonyl compounds

Question 1

descrbe the structure of carbonyl compounds

Answer 1

• carbon-oxygen double bond
• bond is polar due to difference in electronegativity

Question 2

describe the structure of aldehydes

Answer 2

• carbon atom of carbonyl group is attached to 1 or 2 hydrogen atoms
• structual formula functional group = CHO

Question 3

describe the structure of ketones

Answer 3

• carbonyl functional group is joined to 2 carbon atoms
• structural formula functional group = CO

Question 4

how do you name aldehydes?

Answer 4

• carbon atom of carbonyl group is always designated as carbon-1
• suffix is -al

Question 5

how do you name ketones?

Answer 5

• find longest carbon chain for stem
• lowest number suffix where -C=O functional group is found (eg. C2 = -2-)
• ends in -one (eg. pentan-2-one)

Question 6

describe bonding in carbonyl compounds

Answer 6

• overlaps with a 2p orbital of oxygen to form a pi bond
• bond is polar as oxygen is more electronegative than carbon

Question 7

how can aldehydes be formed?

Answer 7

• oxidation of primary alcohols
• RCH₂OH + [O] -> RCHO + H₂O

Question 8

how can ketones be formed?

Answer 8

• oxidation of secondary alcohols
• RCHOHR + [O] -> RCOR + H₂O

Question 9

oxidation of aldehydes

Answer 9

• forms carboxylic acids
• reflux with acidified dichromate (VI) ions, Cr₂O₇²⁻/H⁺
• potassium dichromate (VI), K₂Cr₂O₇ / dilute sulfuric acid, H₂SO₄
• RCHO + [O] -> RCOOH

Question 10

oxidation of ketones

Answer 10

• ketones do not undergo oxidation reactions
• this lack of reactivity provides chemists with a way of distinguishing between aldehydes and ketones

Question 11

reaction of carbonyl compounds with NaBH₄

Answer 11

• NaBH₄ is used as reducing agent to reduce aldehydes and ketones to alcohols
• need H₂O and heat
• [H] = reducing agent

Question 12

reducing an aldehyde

Answer 12

• aldehydes are reduced to primary alcohols by NaBH₄
• RCHO + 2[H] -> RCH₂OH

Question 13

reducing a ketone

Answer 13

• ketones are redued to secondary alcohols by NaBH₄
• RCHOHR + 2[H] -> secondary alcohol

Question 14

reaction of carbonyl compounds with HCN

Answer 14

• add H-CN across C=O in aldehydes and ketones
• useful to increase C chain length
• HCN is too hazardous so NaCN/H₂SO₄ used
• forms hydroxynitrile (eg. 2-hydroxybutanenitrile = CH₃CH₂CH(OH)CN)
• organic product contains -OH hydroxyl group and nitrile group, C≡N

Question 15

mechanism for nucleophilic addition reaction with NaBH₄

Answer 15

• NaBH₄ can be considered as containing the hydride ion, :H-, which acts as the nucleophile

1. lone pair of electrons from hydride ion is attracted and donated to the partial positive C of carbonyl
2. dative covalent bond forms between H and C, pi bond breaks, forming a negatively chargd intermediate
3. lone pair of electrons on O atom is attracted to partial positive H in water molecule and formed a covalent bond
4. intermediate is protonated to form an alcohol
5. hydroxide ion, OH-, is left

Question 16

what kind of molecule is HCN?

Answer 16

• weakly acidic covalent molecule
• very poisonous gas with low bp
• generated from reaction of NaCN/KCN (aq) with sulfuric acid

Question 17

how can a reaction with a carbonyl and NaCN occur?

Answer 17

• weakly acidic solution strengthen polarity in C=O
• resulting solution contains free -CN ions and H+ (aq) from HCN

Question 18

describe the reaction mechanism for NaCN/H+ with a carbonyl

Answer 18

1. lone pair of electrons from cynanide ion, :CN-, is attracted and donated to δ+ carbon atom in carbonyl C=O double bond
2. a dative covalent bond forms
3. pi bond in C=O double bond breaks by heterolytic fission forming a negatively charged intermediate
4. intermediate is protonated by donating a lone pair of electrons to a hydrogen ion to form the product
5. product is a hydroxynitrile