Module 6 - aromatic compounds

Question 1

Describe the reactivity of an alkene

Answer 1

-2 Pi electrons are localised between 2 carbon atoms.
-High electron density
-Polarises a Br2 molecule and attracts it
-Electrophilic addition with a colour change from orange to colourless

Question 2

Describe the reactivity of benzene

Answer 2

-6 Pi electrons are delocalised between 6 atoms.
-Lower electron density than an alkene.
-Only partially polarises a Br2 molecule - a halogen carrier catalyst is needed to generate a more powerful electrophile Br+.
-Undergoes electrophilic subsitution.

Question 3

Describe the reactivity of phenol

Answer 3

-6 Pi electrons are delocalised between 6 carbon atoms.
-A lone pair of electrons in a P orbital on O are delocalised into the ring, activating it.
-Higher electron density than benzene.
-Polarises and attracts a Br2 molecule without a halogen carrier catalyst.
-Electrophilic substitution with a colour change from orange to colourless and a formation of a white precipitate (2,4,6-tribromophenol).

Question 4

describe kekules model of benzene and the evidence that led scientists to doubt the model

Answer 4

a six membered ring structure with alternating single and double bonds
-benzene doesn’t undergo electrophilic addition with bromine like typical alkenes, there is no colour change from orange to colourless when bromine is added
-benzene had a regular planar structure with equal c-c bond lengths, kekules models would have different bond lengths as it included both c=c and c-c bonds
-the actual enthalpy of hydrogenation of benzene is less exothermic than kekules suggested model

Question 5

Describe the structure of an alkene

Answer 5

-sideways overlap of p-orbitals
-pi bond above and below ring
-3 pi bonds are localised
-2 electrons in each pi bond

Question 6

Describe the structure of benzene

Answer 6

-sideways overlap of p-orbitals
-pi bond above and below ring
-delocalised pi ring
-6 electrons in one pi bond

Question 7

define electrophile

Answer 7

electron acceptor

Question 8

define nuceophile

Answer 8

electron pair donator

Question 9

define nuceophile

Answer 9

electron pair donator

Question 10

explain the delocalised module of benzene

Answer 10

-benzene is a planar cyclic molecule withe bond angles of 120 between carbon atoms
-trigonal planar around each carbon atom
-each carbon has three sigma bonds
-the fourth electron is contained in a p orbital
-the p orbitals overlap sideways above and below the plane of the ring to form a pi bond
-six electrons are delocalised over the six carbon atoms

Question 11

compare alkene reactivity with Br2 to benzene’s reactivity with Br2

Answer 11

-in alkenes the pi electrons are localised and there is a high electron density to polarise and attract the Br2 for an electrophilic addition reaction
-in benzene the pi electrons are delocalised giving a lower electron density and benzene can’t polarise and attract a Br2 molecule so can’t undergo electrophilic addition
-benzene can undergo electrophilic substitution with a halogen carrier catalyst

Question 12

describe phenol and it’s uses

Answer 12

-phenols have an OH group directly attached to the benzene ring
-it’s partially soluble in water - hydrogen bonds can be formed between the OH and water molecules, but the benzene ring is non-polar and not soluble in water
-phenol is a weak acid
-uses: used as an antiseptic, in disinfectants, used in the synthesis of dyes

Question 13

Thin layer chromotography

Answer 13

-seperation by adsorption
-continuous monitoring
-measure how far each spot travels and calculate Rf value
-identify componenst by matching their Rf values to known values in a database

Question 14

limitations of thin layer chromatography

Answer 14

-similar compounds will have simila Rf vaalues - so may overlap
-new compounds wont have database values to compare with

Question 15

limitations of gas chromatography

Answer 15

-some compounds have similar retention times
-new compounds wont have database values to compare with

Question 16

define isoelectric point

Answer 16

the ph at which the zwitterion exists

Question 17

define chiral carbon

Answer 17

a carbon attached to four different groups

Question 18

compare the structure of alkene versus benzene

Answer 18

-both have a sideways overlap of p orbitals
-both have a pi bond above and below the ring
-alkene has 3 pi bonds which are localised whereas benzene has a delocalised pi ring
-alkene had 2 electrons in each pi bond whereas benzene has 6 electrons in each pi bond
-alkene has overlap in one direction whereas benzene has overlap in both directions

Question 19

how is phenol able to react with bromine in a electrophilic substitution reaction without a halogen carrier catalyst

Answer 19

-the lone pair on the O atom in phenol which is in a p-orbital is delocalised into the benzene ring, increasing its electron density and activating it
-this greater electron density in the ring can now polarise a Br2 molecule and attract it
-phenol is more elctron dense so multisubstitution occurs
-colour change from orange to colourless and the product 2,4,6 tribromophenol (a white precipitate) occurs

Question 20

describe thevc test for aldehdyes

Answer 20

-add tollens reagent
-if a silver mirror precipitate is formed then an aldehyde is present

Question 21

describe the test for aldehyde and ketones and how this can be used to identify the compound

Answer 21

-add 2,4 DNPH
-an orange/red/yellow hydrazone precipitate shows that a aldehyde or ketone is present
-each hydrazone precipitate has its own unique melting point
-filter, recrystallize, dry find its melting point and match this to a database