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A level- Chemistry > Carbonyls > Flashcards

Carbonyls Flashcards

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1
Q

What are the main carbonyl compounds that are used

A

Ketones, aldehydes

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2
Q

What is the carbonyl functional group

A

C=O

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3
Q

How are aldehydes oxidised and give an example with butanal

A
  1. Refluxed with acidified dichromate ions e.g. potassium dichromate and dilute sulfuric acid
  2. CH3CH2CH2CHO + [O] → CH3CH2CH2COOH
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4
Q

Describe the differences between the C=O and C=C bonds.

A
  1. C=C is non-polar
  2. C=O bond in carbonyl compounds is polar
  3. Oxygen is more electronegative than carbon. The electron density in the double bonds lies closer to the oxygen atom than to the carbon. Making carbon end slightly positive and oxygen end slightly negative
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5
Q

Explain what affect the C=O bond in carbonyls has on their chemical reactions

A
  1. Because of the polarity of the C=O bond, aldehydes and ketones react with some nucleophiles
  2. A nucleophile is attracted to and attacks the slightly positive carbon atom resulting in addition across the C=O bond
  3. This is nucleophilic addition
  4. Different from non-polar C=C bond in alkenes which reacts with electrophiles in electrophilic addition
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6
Q

Explain in terms of the type and strength of intermolecular forces, the difference in the boiling points of ethanol (78.4 degrees), ethanal (20.2 degrees) and chloromethane (-24.2 degrees).

A
  1. -The OH group in an alcohol has a H bonded to the highly electronegative oxygen and lone pairs of electrons.
    - This allows intermolecular hydrogen bonding and increases the boiling point.
    - It also has London forces.
  2. -The aldehyde group has lone pairs of electrons but no electron-deficient hydrogens, so it cannot hydrogen bond with other aldehyde molecules.
    - However it contains the highly polar aldehyde group allowing dipole-dipole interactions and is linear allowing relatively strong London forces, giving it an intermediate boiling point.
  3. -Chloromethane cannot form hydrogen bonds, is less polar than ethanal, and its shape also means that it will form weaker London forces, giving it the weakest intermolecular forces and the lowest boiling point.
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7
Q

What is a common reducing agent used in the reduction of aldehydes and ketones

A

NaBH4

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8
Q

What are the conditions for reducing an aldehyde or ketone

A
  1. Aldehyde or ketone is usually warmed with NaBH4 in aqueous solution
  2. Usually 2[H]
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9
Q

Write the equation for the reaction of NaBH4 with the following compounds:
Butanal, butanedione and propanone

A
  1. CH3CH2CH2CHO + 2[H] → CH3CH2CH2CH2OH (butan-1-ol)
  2. CH3COCOCH3 + 4[H] → CH3CH(OH)CH(OH)CH3
  3. CH3COCH3 + 2[H] →CH3CH(OH)CH3
  4. NaBH4/H2O should be written above the arrow
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10
Q

Describe the properties of hydrogen cyanide and what is used as a replacement in the laboratory

A
  1. Colourless
  2. Extremely poisonous
  3. Sodium cyanide and sulfuric acid are used to provide the hydrogen cyanide
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11
Q

Describe the reaction between HCN and aldehydes/ ketones

A
  1. HCN adds across the C=O bond
  2. An addition reaction
  3. Provides a means of increasing the length of he carbon chain
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12
Q

What does the organic product of the reaction between HCN and aldehyde/ketone look like and what are they classed as

A
  1. Contains two functional groups- OH and C(triple bond)N

2. These compounds are classified as hydroxynitriles

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13
Q

Write the equation for the reaction between HCN and:
1. propanal, 2. Propanaone
And name the products

A
  1. CH3CH2CHO + HCN → CH3CH2CH(OH)CN 2-hydroxybutanenitrile
  2. CH3COCH3 + HCN → CH3C(OH)CNCH3 2-hydroxy-2-methylpropanenitrile
  3. Above the arrow should be H2SO4/NaCN
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14
Q

Draw dot and cross diagrams for CN- and HCN

A
  1. CN- 3 shared pairs of electrons, nitrogen and carbon have a lone pair each. One of the electrons in the carbon lone pair is from a different species. Should be enclosed in square brackets with minus sign
  2. HCN- 3 shared pairs between C and N, N has a lone pair, a shared pair between C and H, no charge
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15
Q

Explain whether CN- or HCN could both act as nucleophiles and if so which would be likely to be the better nucelophile

A
  1. Both could act as nucleophiles as they each have lone pairs of electrons
  2. CN on both C and N
  3. HCN on only the N
  4. CN- has 2 lone pairs and a negative charge so is more likely to be able to act as a nucleophile
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16
Q

Describe and name the mechanism for the reaction of an aldehyde/ketone with NaBH4

A
  1. Nucleophillic addition
  2. NaBH4 can be considered as containing the hydride ion which acts as the nucleophile
  3. The lone pair of electrons from the hydride ion is attracted and donated to the delta+ carbon atom in the aldehyde or ketone C=O double bond.
  4. A dative covalent bond is formed between the hydride ion and the carbon atom of the C=O bond.
  5. The pi-bond in the C=O bond breaks by heterolytic fission forming a negatively charged intermediate
  6. The oxygen atom of the intermediate donates a lone pair of electrons to a hydrogen atom in a water molecule.
  7. The intermediate has then been protonated to form an alcohol.
17
Q

Draw the mechanism for the reaction between propanone and NaBH4

A
  1. Start with propanone with delta + charge on carbon and delta- charge on oxygen of C=O bond
  2. Hydride ion with a negative charge an lone pair of electrons
  3. Draw curly arrows from the lone pair of electrons from the hydride ion to the delta+ carbon atom and a curly arrow from the C=O bond to the delta- oxygen atom
  4. This forms intermediate- the H is attached to the carbon and the oxygen now has a lone pair of electrons and negative charge
  5. A curly arrow is drawn from the lone pair of electrons on the oxygen atom to a water molecule with its delta+/- charges
  6. A curly arrow is also drawn from the H-O bond in the water to the O.
  7. This forms the product of CH3CH(OH)CH3 + OH-
18
Q

Describe and name the mechanism for the reaction of an aldehyde/ketone with NaCN/H+

A
  1. Nucleophilic addition
  2. The CN- acts as the nucleophile
  3. The lone pair of electrons from the CN- ion (the lone pair on the carbon) is attracted and donated to the delta+ carbon atom in the aldehyde/ketone C=O double bond
  4. This forms a dative covalent bond
  5. The pi-bond in the C=O double bond breaks by heterolytic fission forming a negatively charged intermediate
  6. The intermediate is protonated by donating a lone pair of electrons to a hydrogen atom to form the product.
  7. The product is a hydroxynitrile
19
Q

Draw the mechanism for the reaction of propanal with sodium cyanide in the presence of a acid

A
  1. Start with propanal with delta + charge on carbon and delta- charge on oxygen of C=O bond
  2. Cyanide ion with a negative charge and a lone pair of electrons
  3. Draw curly arrows from the lone pair of electrons from the cyanide ion to the delta+ carbon atom and a curly arrow from the C=O bond to the delta- oxygen atom.
  4. This forms intermediate- the CN is attached to the carbon and the oxygen now has a lone pair of electrons and negative charge
  5. A curly arrow is drawn from the lone pair of electrons on the oxygen atom to a hydrogen ion. (Water can also be used as it is in the reaction with NaBH4)
  6. This forms the product- CH3CH2CH(OH)CN
20
Q

Describe how to name hydroxynitriles

A
  1. Start with the C attached to the N - this is carbon one
  2. Then the OH groups are the hydroxy groups and are written first with a number to show which carbon they are attached.
  3. Then any methyl groups are written
  4. The carbon bit is written with an -e and nitrile after e.g. propanenitrile or butanenitrile
21
Q

State two reagents/chemicals that can be used to identify a carbonyl compound

A
  1. 2,4-dinitrophenylhydrazine (2,4-DNPH) / Brady’s reagent

2. Tollen’s reagent (ammoniacal silver nitrate)

22
Q

Describe what 2,4-DNPH can be used to identify

A
  1. Used to detect the presence of the carbonyl functional group in aldehydes and ketones
  2. In the presence of a carbonyl group a yellow or orange precipitate is formed
23
Q

Describe how to use 2,4-DNPH in practical work

A
  1. 2,4-DNPH is normally dissolved in methanol and sulfuric/phosphoric acid as a pale orange solution.
  2. Add this solution to the unknown compound and stir with glass rod
  3. Orange precipitate indicates presence of aldehyde or ketone
24
Q

Describe what Tollen’s reagent can be used to identify

A
  1. Once a compound has been identified as containing a carbonyl group, Tollen’s can identify whether it is a ketone or aldehyde
  2. In presence of aldehyde a silver mirror is produced.
25
Q

Describe how to use Tollen’s reagent in practical work

A
  1. Tollen’s reagent has short shelf-life and should be made immediately before carrying out the test.
  2. Mix aqueous silver nitrate with aqueous sodium hydroxide until a brown precipitate of silver oxide is formed
  3. Add dilute ammonia solution until the brown precipitate dissolves to form a clear colourless solution of Tollen’s reagent
  4. Then add the unknown substance and heat in hot water bath.
  5. Silver mirror indicates presence of aldehyde
26
Q

Show reaction of Tollen’s reagent reaction with an aldehyde

A
  1. Tollen’s reagent contains silver ions which act as an oxidising agent in the presence of ammonia
  2. In the reaction silver ions are reduced to silver as the aldehyde is oxidised to a carboxylic acid
  3. Ag+ + e- → Ag (s)
  4. Aldehyde + [O] → carboxylic acid
27
Q

How can you identify the carbonyl compound after a 2,4-dinitrophenylhydrozone precipitate has been formed

A
  1. The impure orange solid is filtered to separate the solid precipitate from the solution.
  2. The solid is then recrystallised to produce a pure sample of crystals
  3. The melting point of the hydrozone is measured and recorded
  4. The melting point is then compared to a database or data table of melting points to identify the original carbonyl compound
28
Q

What is a nitrile group

A

Organic functional group -CN

29
Q

What can nitriles be formed from

A
  1. Haloalkanes

2. Aldehydes and ketones

30
Q

Describe how nitriles can be formed from haloalkanes

A
  1. Reacting haloalkanes with sodium cyanide, NaCN or potassium cyanide, KCN, in ethanol
  2. This increases the length of the carbon chain
31
Q

Write the equation and draw the mechanism for the reaction of 1-chloropropane with KCN

A
  1. CH3CH2CH2Cl + KCN → CH3CH2CH2CN + KCl
  2. A nucleophilic substitution mechanism
  3. Curly arrow from lone pair of electrons in negative carbon atom of CN to delta+ carbon atom of C-Cl bond
  4. Curly arrow from C-Cl bond to delta negative chlorine atom
  5. CN takes place of Cl leaving a Cl ion
32
Q

Describe the reduction reaction of nitriles and give example with propanenitrile

A
  1. Nitriles can be reduced to amines by reacting with hydrogen in the presence of a nickel catalyst
  2. CH3CH2C≡N + 2H2 → CH3CH2CH2NH2
  3. Propanenitrile + hydrogen → propylamine
33
Q

Describe the hydrolysis of nitriles and give example with butanenitrile

A
  1. Nitriles undergo hydrolysis to form carboxylic acids by heating with dilute aqueous acid e.g HCl (aq)
  2. CH3CH2CH2≡N + 2H2O + HCl → CH3CH2CH2COOH + NH4Cl
  3. Butanenitrile + water + hydrochloric acid → butanoic acid + ammonium chloride

A level- Chemistry (31 decks)

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