carbonyl compounds and identifying aldehydes and ketones

Question 1

What compounds contain the carbonyl functional group

Answer 1

• aldehydes
• ketones

Question 2

What is the carbonyl functional group

Answer 2

C=O

Question 3

Describe the structure of an aldehyde

Answer 3

• the carbonyl functional group is found at the end of a carbon chain
• the carbon atom of the carbonyl group is attatched to one or two hydrogen atoms

Question 4

What is the structural formula of the aldehyde group

Answer 4

CHO

Question 5

What is the formula of the simplestt aldehyde

Answer 5

• HCHO

Question 6

Describe the structure of a ketone

Answer 6

• the carbonyl functional group is joined to two carbon atoms in a carbon chain

Question 7

What is the structural formula of a ketone group

Answer 7

CO

Question 8

what is the simplest ketone and give the formula

Answer 8

CH3COCH3
propanone

Question 9

What is another word for methanala and its use

Answer 9

• formaldehyde
• used in solution to preserve biological specimens

Question 10

What is another name for propanone and give its use

Answer 10

• acetone
• important in industrial solvent and used in nail varnish remover

Question 11

What is the suffix of an aldehyde group when naming them

Answer 11

al

Question 12

What is the suffix of a ketone when namine them

Answer 12

one

Question 13

Describe the oxidation of aldehydes

Answer 13

• adehydes can be oxidised to carboxylic acids when refluxed with acidified dichromate (VI) ions Cr2O72-/H+ usually as a mixture of sodium or potassium dichromate (VI) K2Cr2O7 and dilute sulfuric acid H2SO4

Question 14

What is a way of distinguishing ketones from aldehydes

Answer 14

• using potassium dichromate
• aldehyde will turn green
• ketones will show no change as they do not undeergo oxidation reactions

Question 15

Describe the difference of reactivity between carbonyl and alkene functional groups

Answer 15

• C=C double bond in alkenes is non polar
• C=O double bond in carbonyl is polar

Question 16

Describe the reactivity of aldehydes and ketones

Answer 16

• it is influenced by the nature of the carbon oxygen double bond
• oxygen is more electronegative than carbon
• the electon density in the double bond lies closer to the oxygen atom than the carbon
• makes the carbon end of the bond slightly positive and the oxygen ened slightly negative
• due to the polarity of the C=O double bonds aldhydes and ketons react with ome nucleophiles
• nucleophile is attracted to and attacks the slightly positive carbon atoom reslting in addition across C=O double bond
• reaction is nucleophillic addition

Question 17

What is NaBH4 and what reaction is it used for

Answer 17

• used as a reducing afent to reduce aldehydes and ketones to alcohols

Question 18

Describe how you turn an aldehyde or a ketone into a into an alcohol

Answer 18

• warming the aldehyde or ketone with NaHB4 reducing agent in an aqueous solution

Question 19

What are aldehydes reduced to

Answer 19

primary alcohols

Question 20

W

What are ketones reduced to

Answer 20

secondary alcohols

Question 21

Describe the reaction of carbonyl compounds with hydrogen cyanide

Answer 21

• hydrogen cyanide HCN adds across the C=O double bond of aldeydes and ketones
  *

Question 22

Describe why NACN used when reacting HCN with carbonyl compounds

Answer 22

• HCN is colourless and extremely poisonous liquid that boils slightly above room temperature so cannot be used safely in an open larbatory
• sodium ccyanide and sulfuric acid are used to provide the hydrogen cyanide in the reaction

Question 23

What is the product of an aldehyde reacting with HCN and what are the functioncal groups

Answer 23

• hydroxynitrile
• -OH
• Ctriple bond N

Question 24

Why is the reaction of carbonyl compounds and HCL useful

Answer 24

• increasing the length of the carbon chain

Question 25

What does the carbon atom in the C=O bond do

Answer 25

* it is electron defifcient attracting nucleophiles

Question 26

What is the reaction mechanism for aldehydes and ketones

Answer 26

nucleophillic addition

Question 27

Describe the mechanism reaction with NaBH4 with a carbonyl group

Answer 27

* the lone pair of electrons from the hydride ion H- is attracted and donated to the positive dipole on the carbon atom in the aldehyde or ketone C=O double bond * a dative covalent bond is formed between the hydride ion and the carbon atom of the C=O double bond * the pi bond in the C=O double bond breaks by heterolytic fission forming a negativley charged intermediate * the oxygen atom of the intermediate donates a lone pair of electrons to a hydrogen atom in a water molecule the intermediate has then been protatnated to form an alcohol

Question 28

Describe the mechanism for the reaction with NaCn/H+

Answer 28

* the cyanide attacks the electron deficient carbon atom in the aldehyde or ketone * the lone pair of electrons is attracted and donated to the partial positive chargew on the carbon atom in the aldehyde or ketone C=O double bond a dative covalent bond forms * the pi bond in the C=O double bond breaks via heterolytic fission forming a negativley charged intermediate * the intermediate is protonated by donating a lone pair of electrons to a hydrogen ion to form the product * the product is a hydroxynitrile

Question 29

What is used to detect a carbonyl group

Answer 29

* a solution of 2,4-dinitrophenylhydrazine 2,4-DNP also known as bradys reagent is used to detect the carbonyl functional group * a yellow or orange precipitate called 2,4-diitrophenylhydrazone is produced

Question 30

Why is the solid 2,4-DNP dangerous and how do we use it in practicals

Answer 30

* normally used dissolved in methanol and sulfuric acid * can be hazardous because of friction or a sudden blow can cause it to explode

Question 31

outline the method for testing for a carbonyl group

Answer 31

* add 5cm depth of a solution of 2,4-dinitrophenylhydrazine to a clean test tube. This is in excess * using a dropping pipette add three drops of unknown compounds leave to stand * if no crytals form add a few drops of sulfuric acid * a yellow/orange preipitate indicates the prescence of an aldehyde or ketone

Question 32

Describe the test to distinguish between aldehydes and ketones

Answer 32

* tollens reagent * solution of silver nitrate in aqueous ammonia * in prescence of an aldehyde group a silver mirror is produced

Question 33

Describe the method for testing for an aldehyde group

Answer 33

* in a clean test tube add 3cm depth of aqueous silver nitrate AgNO3(aq) * add aqueous sodium hydroxide to the silver nitrate until a brwon precipitate of silver oxide Ag2Ois formed * add dilute ammonia solution until the brown precipitate just dissolves to form a clear colourless solution - this is tollens reagent * pour 2cm depth of the unknown solution into a clean test tube * add equal volume of freshly prepared tollen reagent * leave the test tube to stand in a beaker of warm water for about 10 to 15 minutes and observed wether any silver mirror is formed

Question 34

Describe how tollens reagent identifies ann aldehyde

Answer 34

it contains silver ions which act as an oxidising agent in the precence of ammonia solver ions are reduced to silver as the aldehyde is oxidised to a carboxylic acid

Question 35

Describe the method for identify an aldehyde or a ketone by melting point

Answer 35

* impure yellow/orange solid is filtered to seperate the solide precipitate from the solution * the solid is then recrystallied to produce a pure sample of crystals * the metling point of purified 2,4-dinitrophenylhydrazone is measured and recorded * the melting pointd id then compared to a database or data table of melting point to identify the original carbonyl compoud