Carbonyls

Question 1

What is a carbonyl bond?

Answer 1

C=O

Question 2

Describe a carbonyl bond.

Answer 2

Carbonyl bonds form sigma and pi bonds in the same way that C=C bonds do, with the overlap of p orbitals. They are polar due to the electronegativity of oxygen.

Question 3

WHat is the shape around a C atom in a carbonyl bond?

Answer 3

Trigonal planar with a 120 degree bond angle.

Question 4

What is the functional group of an aldehyde?

Answer 4

-CHO

Question 5

What is the functional group of a ketone?

Answer 5

-CO-

Question 6

What is the functional group of an ester?

Answer 6

-COO-

Question 7

What is an acyl chloride?

Answer 7

A carboxylic acid with a chlorine atom instead of an OH group

Question 8

What is the functional group of an acyl chloride?

Answer 8

-COCl

Question 9

What is Brady’s reagent?

Answer 9

2, 4- DNPH dissolved in phosphoric acid and methanol

Question 10

Describe the appearance of Brady’s reagent.

Answer 10

A clear orange solution.

Question 11

How does Brady’s reagent react with carbonyls?

Answer 11

Carbonyl compounds form an orange precipitate (a 2,4-DNPH derivative) in Brady’s reagent.

Question 12

What is the test for a carbonyl compound?

Answer 12

Brady’s reagent.

Question 13

How can 2,4-DNPH derivatives be used to identify carbonyls?

Answer 13

They have varying melting points.

Question 14

What is the test for aldehydes?

Answer 14

Tollens’ reagent.

Question 15

What is tollens’ reagent?

Answer 15

Ammoniacal silver nitrate.

Question 16

How does tollens’ reagent react with aldehydes?

Answer 16

It oxidises them to carboxylic acids, and the reduced silver deposits on the test tube in a silver mirror.

Question 17

Why does tollens’ reagent not react with alcohols?

Answer 17

It is not a sufficiently strong oxidising agent.

Question 18

What is NaBH4?

Answer 18

A reducing agent

Question 19

How does NaBH4 act as a reducing agent for carbonyls?

Answer 19

It releases hydride ions (H-) which attack the delta+ charge in the carbonyl bond.

Question 20

Name the mechanism for the reduction of carbonyls.

Answer 20

Nucleophilic addition.

Question 21

Describe the mechanism of the reduction of a carbonyl.

Answer 21

A hydride ion donates an electron pair and bonds to the C delta+ of a carbonyl bond and the C=O bond splits heterolytically leaving a single bonded O- ion. The O- ion donates an electron to bond with an H delta+ from water, and an OH- ion from that water molecule is left over.

Question 22

How does reduction affect a carbonyl bond?

Answer 22

It changes from C=O to C-OH

Question 23

Why can reduction not occur in acidic solution?

Answer 23

The hydride ion would react with an H+ ion to give hydrogen, rather that reducing the carbonyl.
H- + H+ —–> H2

Question 24

What is a cyanide ion?

Answer 24

CN-

A carbon and nitrogen atom, triple bonded, where the C atom’s lone pair contains a borrowed electron

Question 25

What is hydrogen cyanide?

Answer 25

H+ with CN-

Question 26

Which atom in a cyanide ion acts as a better nucleophile?

Answer 26

The carbon atom as it is less electronegative than nitrogen.

Question 27

Describe the reaction mechanism between a carbonyl and a cyanide ion.

Answer 27

The C atom from cyanide donates a lone pair to the C delta+ in the carbonyl bond to form a new covalent bond. The pi bond from the C=O group spilts heterolytically, leaving a singly bonded O- group. The O- group donates an electron pair to an H+ ion to form a new -OH group.

Question 28

What is the product of the reaction between CN- and a carbonyl.

Answer 28

A hydroxy nitrile with an extended carbon chain.

Question 29

Why is the addition reaction of CN- and C=O important in synthesis?

Answer 29

It allows chemists to extend the carbon chain of molecules.

Question 30

What type of molecule is hydrogen cyanide?

Answer 30

A weak acid. | HCN H+ + CN-

Question 31

How does [H+] affect the abundance of CN- in a reaction.

Answer 31

It reduces the concentration of CN- as it causes the equilibrium of the weak acid to shift towards HCN, removing H+ and CN- from the solution.

Question 32

How does [H+] affect the rate of reaction Between -O- ions and H+.

Answer 32

A higher concentration of H+ ions increases reaction rate

Question 33

Under what conditions does the addition of CN- with C=O usually take place?

Answer 33

CN- salt is used (eg. KCN) and a small amount of acid is added.

Question 34

What is acid hydrolysis?

Answer 34

The process of converting a nitrile group to a carboxylic acid.

Question 35

What are the conditions for acid hydrolysis?

Answer 35

Hot aqueous HCl.

Question 36

Give a general equation for acid hydrolysis.

Answer 36

Nitrile + 2H2O + HCl -----> Carboxylic acid + NH4Cl

Question 37

Give a general equation for the reduction of nitriles.

Answer 37

Nitrile + 4[H] -----> Amine

Question 38

What is an amine group?

Answer 38

NH2

Question 39

What are the conditions for the reduction of nitriles?

Answer 39

Hydrogen and a nickel catalyst.

Question 40

WHat is the mechanism of the reaction between haloalkanes and CN-?

Answer 40

Nucleophilic substitution.

Question 41

Describe the mechanism of the reaction between haloalkanes and CN-.

Answer 41

The CN- nucleophile attacks the polar C-X bond, forming a new covalent bond between the two C atoms to give a nitrile and displacing an X- ion.