Topic 17 - Organic chemistry (17.1 To 17.3)

Question 1

Define…

1) structural isomers
2) functional group isomers
3) chain isomers
4) positional isomers
5) stereoisomers
6) geometric isomers

Answer 1

1) compounds with same molecular formula but different structural formulae
2) different functional groups e.g aldehydes, ketones
3) different patterns of branching in their carbon chains
4) same functional group is attached to different carbon atoms in the chain
5) have same structural formulae but atoms or groups are arranged differently in three dimensions
6) atoms or groups are attached at different positions on opposite sides of a C=C double bond

Question 2

Define chirality and chiral

Explain terms of it what it is

Answer 2

Refers to an atom in s molecule that allows it to exist as non superimposable form
Molecule consists of single carbon atom joined to four different groups or atoms - represent this with asterisk - asymmetric refers to carbon atom in molecule that is joined to four different atoms or groups

Question 3

What is an enamtiomer

Answer 3

Isomers that are related as object and mirror image - also if molecule has chiral centre then exists as optical isomers

Question 4

What is plane-polarised light?

Answer 4

Monochromatic light that has oscillations in only one plane

Question 5

What is polarimetry?

How does it work?

Answer 5

Polarimetry is use of a polarimeter to measure the amount of optical activity (rotates plane polarised light)
Monochromatic light source passes through polarising filter called the polariser as converts unpolarised light to plane polarised light. Then passes through sample tube containing some of the substance in solution - if optically active because contains an enantiomer then plane of polarisation will be rotated so no longer vertical. If clockwise then dextrorotatory or anti clockwise laevorotatory
Second polarising filter analyser allows plane polarised light to pass through it - rotated to a position where maximum light intensity can be seen - angle of rotation is measured

Question 6

Describe properties of enantiomers

Answer 6

Two enantiomers have identical physical properties except the enantiomers rotate the plane of polarisation of plane polarised light by equal angles but in opposite directions.
Two enantiomers have identical chemical properties with one exception the way in which they rest with other substances

Question 7

What is a racemic mixture?

Answer 7

Mixture containing equal amounts of two enantiomers

Question 8

Show the SN2 and SN1 mechanism

Evidence for optical activity for both

Answer 8

SN2 = bimolecular - two species involved in the rate-determining step
HO attacked C attached to CH3, F, H, Br and arrow from C —- Br
Goes to in brackets with - C attached to HO, CH3, Br, F and H
Then goes to C bonded to everything without Br and Br- on the side

SN1 = uni molecular - one species in rate-determining step

1) Arrow from C to Br
2) C+ attached to CH3 and F and H
3) OH- attack from either side so HO group attached either on left or right

In SN1 - original halogenoalkane has tetrahedral shape but carbocation has planar shape - second step of reaction has equal chance for attacking HO- to approach from either the left or the right so there are two products which are enantiomers - present in equal numbers so racemic mixture formed

Question 9

General difference between an aldehyde and a ketone

Answer 9

Aldehydes carbonyl compound - hydrogen atom joined to the carbonyl group - suffix is al. E.g. HCHO = methanal
Ketones only hydrocarbon groups joined to the carbonyl - suffix one e.g. CH3COCH3 = propanone

Question 10

properties of aldehydes/ketones

1) boiling temperatures
2) solubility
3) bonding
4) physical

Answer 10

1) Contain polar C=O group so have dipole-dipole interactions but do not have hydrogen bonds because all of the hydrogen atoms are joined to carbon atoms. Boiling temperatures increase as increasing molar mass so London forces increases.
2) lower aldehydes/ketones double in water because it can form hydrogen bonds with water molecules but solubility decreases with increasing chain length
3) C=C double bond has sigma and pi bonds. Bond in carbonyl is polar because of differing electronegativities of C and O. Electron density in pi bond is greater nearer the oxygen atom
4) distinctive smells - short carbon chains unpleasant whereas long have pleasant and used in perfumes.

Question 11

Describe reduction reactions for carbonyl compounds

Answer 11

Both aldehydes and ketones can be reduced by reagent lithium tetrahydridoaluminate - LiAlH4 - reducing agent dissolved in dry ether. Aldehydes form primary alcohol and ketones form secondary alcohol
CH3CH2CHO + 2[H] –> CH3CH2CH2OH = propan-1-ol

Question 12

Describe oxidation reactions of carbonyl groups

State the reagent, colour changes and any observations/reasons for colour changes

Answer 12

1) acidified potassium dichromate (VI) - goes from orange solution to green solution
Colour change due to reduction of dichromate ions (Cr2O7)2- to chromium ions from oxidation 6+ to 3+
2) Fehlings/Benedicts solution - deep blue solution to red precipitate
Conversion of Copper(II) complex to copper(I) oxide
3) Tollens reagent - colourless solution to silver mirror - conversion of silver(I) complex to metallic silver which sticks to inside of tube. Reagent is [Ag(NH3)2]+

Question 13

Distinguishing between aldehydes and ketones from oxidation reactions

Answer 13

A positive result indicates presence of aldehydes so if negative generally a ketone

Question 14

Describe the reactions with iodine?

Process, colour changes ect.

Answer 14

Redox reaction - iodoform reaction
Carbonyl compound added to alkaline solution of iodine and mixture warmed and then cooled - pale yellow precipitate forms - CHI3 yellow insoluble solid

Question 15

Describe nucleophilic addiction reaction with hydrogen cyanide - give mechanism and state using optical activity as evidence

Answer 15

Reagent is hydrogen cyanide - reaction carried out in aqueous alkaline solution containing potassium cyanide
CH3CH2CHO + HCN —> CH3CH2CH(OH)CN
Addition reactions involved hydrogen atom attaching to oxygen atom and cyanide group attaching to carbon atom
Two step mechanism
Step 1) NC- arrow to C and arrow from double bond to O - nucleophilic attack forming intermediate CN, H, CH3 and O- bonded to C
Step 2) Arrow from O- to H and arrow from H–CN to form C bonded to a CN, OH, CH3 and H + CN-

Arrangement of the two atoms or groups joined to C=O is planar which means in step 1 equal chance of cyanide ion attacking from each side of plane. So although product contains chiral centre, equal amounts of the enantiomers to form giving a racemic mixture - no optical activity.

Question 16

Describe reaction of 2,4 - dinitrophenylhydrazine

Answer 16

Also known as Brady’s reagent - 2,4 DNPH
Reagent reacts with carbonyl compound to form a bright Orange/yellow precipitate indicating presence of a carbonyl compound

Question 17

Describe properties of carboxylic acids

1) Bonding
2) Physical properties
3) boiling temperature
4) Solubility in water

Answer 17

1) Three polar bonds present so many relations involve loss of hydrogen atom and producing carboxylate ion in which two carbon oxygen bonds are identical
2) Distinctive smells/sour tastes e.g. Citric acid give lemons sour taste
3) presence of three polar bonds including polar O-H bond means strong intermolecular forces because there are hydrogen bonds. So high bolide get temperatures and London forces increase between non polar chains increasing
4) Shorter chain are soluble as form hydrogen bonds but decreases as hydrocarbon part becomes larger

Question 18

Describe preparation by oxidation

Answer 18

Oxidation sues either primary alcohol or aldehyde as starting material - oxidising agent is acidified potassium dichromate and heat mixture under reflux - reaction is fractionally distilled for pure carboxylic acid

Question 19

Preparation by hydrolysis - acidic and alkaline

Answer 19

Nitriles containing CN group - hydrolysed by heating under reflux with either acid/alkali - C Triple bond N breaks and carbon atom part of organic product and nitrogen atom either ammonia/ammonium ion
1) Propanoic acid prepared from propane nitrile
CH3CH2CN + H+ + 2H2O -> CH3CH2COOH + NH4+
2) Butanoic acid prepared from butanenitrile
CH3CH2CH2CN + OH- + H2O -> CH3CH2CCH2COO- + NH3

Question 20

Four reactions of carboxylic acids

1) Reduction
2) Neutralisation
3) Halogenation
4) esterification

Answer 20

1) Carboxylic acids reduced to primary alcohols but not to aldehydes. Reducing agent is Lithium tetrahydridoaluminate and used in solvent dry ether e.g. Butanoic acid forms butan-1-ol
2) Neutralised by mixing with aqueous alkali - products are carboxylate salts e.g. Sodium ethanoate CH3COOH + NaOH -> CH3COONa + H2O
3) OH group is replaced by halogen atom - products known as acto chlorides. Reagents is phosphorus(V) chloride and used in anhydrous conditions.
CH3CH2COOH + PCl5 -> CH3CH2COCl + POCl3 + HCl
4) Carboxylic acid mixed with alcohol and small amount of acid catalysis usually concentrated sulfuric acid - esters used in industry
HCOOH + CH3CH2OH –> HCOOCH2CH3 + H2O

Question 21

Describe reactions of acyl chloride

1) water
2) alcohols
3) concentrated ammonia solution
4) amines

Answer 21

1)acyl chlorides react with cold water forming carboxylic acid and releasing hydrogen chloride gas - misty white fumes
2) React with ethanol to form an ester and hydrogen chloride gas.
3) H of second reactant combines with CL to form HCl and other part of reactant becomes carbonyl group - NH2 group joined to carbonyl group produces amide - CH3COCL + 2NH3 –> CH2CONH2 + NH4Cl
4) Substitued amide - N there is an alkyl group attached
CH3COCL + (CH3)2NH –> CH3CON(CH3)2 + HCl

Question 22

Physical properties of esters

Answer 22

Colourless liquids with low melting/boiling temperatures and insoluble in water
All hydrogen toms attached to carbon atoms so no hydrogen bonds
Pleasant smells so used in perfumes, food flavourings

Question 23

Hydrolysis of esters - in acidic and alkaline solutions

Answer 23

CH3COOH + CH3CH2OH –> CH3COOCH2CH3 + H2O
Ethyl ethanoate formed from carboxylic acid and alcohol
Acidic - warming ester using catalyst of sulfuric acid will speed up hydrolysis but won’t affect equilibrium position
Alkaline - produces carboxylate salt - reaction goes to completion

Question 24

What is saponification and describe the process

Answer 24

Soap making. Alkaline hydrolysis of esters - vegetable oils contain large quantities of triglycerides - trimesters consists of large ester molecule and hydrolysed to one alcohol and three carboxylic acid molecules e.g. Triglyceride - ester of long carboxylic acids and trio forms propane-1,2,3 trill and sodium octadecanoate

Question 25

What is…

1) addition polymerisation
2) condensation polymerisation

Answer 25

1) monomer propene combine with other propene to form long chain
2) formation of a polymer by two different monomers - small molecule is formed. Normally water

Question 26

What is polyesters in terms of condensation polymerisation

Answer 26

Once one molecule of an alcohol has reacted with one molecule of carboxylic acid, reaction is complete - only organic product is slightly larger ester molecule
For reaction to produce polymer, we need 2 monomers each with two reactive groups at either end - alcohol usually a diol with two hydroxyl groups H-O-CH2-CH2-O-H
Dicarboxylic acid commonly used like terephthalic acid - OH of carboxylic acid and H of alcohol react to form small molecule of water and two molecules linked by an ester group. Larger molecule formed still AHS reactive groups at both ends so can continue to react with other molecules until long polymer chain has formed