chemistry: topic 18

features of arene compounds

features of arene compounds

• contain a benzene ring

• high melting points as a result of the benzene ring

• low boiling points as they’re non-polar molecules

• cannot be dissolved in water

structure of benzene

• ring of delocalised electrons

• outer electron from the p-orbital is delocalised in the centre to form the central ring

• this forms pi bonds

• this makes benzene very stable compared to other molecules of a similar size

thermodynamic stability of benzene

• enthalpy of hydrogenation of cyclohexane: -119kJmol-1

  with one double bond

• so theoretically benzene should be triple that, at -357kJmol-1

• however it is only -208kJmol-1

• therefore benzene is more thermodynamically stable and has a different structure to that of cyclohexane

bond lengths in benzene

• all bond lengths between carbon atoms in benzene are the same

• if the cyclohexatriene structure was correct, three bond lengths would be shorter than the other three as they would be double bonds, causing distortion

why is benzene resistant to electrophilic addition reactions, eg bromination?

because addition mechanisms cause permanent disruption to the delocalised system in the centre of the benzene ring and its

substitution mechanisms cause momentary disruption, after which the ring is restored

electrophilic substitution of benzene

• high electron density in the delocalised ring of benzene makes it susceptibe to attack from electrophiles

reagents in nitration of benzene

concentrated nitric acid to produce the NO2+ ion

concentrated sulfuric acid to act as the catalyst, so it not used up in the reaction

conditions in nitration of benzene

reflux at 55*

not higher or it will become explosive!

what is the electrophile in nitration of benzene?

nitronium ion

NO2+

generated in an acid base reaction

2H2SO4 + HNO3 ⇌ 2HSO4- + H3O+ + NO2+

use of nitration of benzene reaction

formation of dyes and explosives

draw a nitration of benzene mechanism

reagents in halogenation of benzene

chlorine

halogen carrier

conditions of halogenation of benzene

reflux in the presence of a catalyst (AlCl3, FeBr3)

anhydrous conditions

chlorine is nonpolar and not a good electrophile so a halogen carrier needs to polarise the hydrogen

draw a halogenation of benzene reaction mechanism

what is the electrophile in the halogenation of benzene and how is it generated?

the halogen

eg Br+ in FeBr3 of Cl- in AlCl3

eg for FeBr3

iron (III) bromide polarises the Br2 molecule, making it easier for the bromine bond to break so that the bromine atom can act as an electrophil

what occurs during a friedel-crafts acylation reaction

delocalised electron ring in benzene acts as a nucleophile, leading to nucleophilic attack on acyl chlorides

conditions required for a fridel-crafts acylation reaction

reflux at 50*

dry inert solvent (dry ether)

what must be produced for a friedel-crafts acylation reaction to occur and how is it done?

a reactive intermediate must be produced from a reaction between the acyl chloride and aluminium chloride catalyst

R-COCl + AlCl3 → R-CO+ + AlCl4-

how does a friedel-crafts acylation reaction progress?

• reactive intermediate produced previously is attacked by the benzene ring of delocalised electrons

• the H+ ion is removed from the ring and reacts with the AlCl4- ion to reform the AlCl3

• hence AlCl3 is a catalyst

• produces a phenylketone (benzene being the phenyl group)

what is the electrophile during fridel-crafts alkylation and how is it formed?

a carbocation

C2H5Cl + AlCl3 → AlCl4- + C2H5+

conditions required for a fridel-crafts alkylation reaction

room temperature

dry inert solvent (dry ether)

reagents required for a fridel-crafts acylation reaction?

acyl chloride and anhydrous AlCl3

reagents required for a fridel-crafts alkylation reaction

anhydrous AlCl3

haloalkane

overall equation for a fridel-crafts alkylation reaction

C6H6 + C2H5Cl → C6H5C2H6 + HCl

draw the mechanism for a fridel-crafts alkylation reaction

draw the mechanism for a fridel-crafts acylation reaction

phenol reaction with bromine water

reacts via multiple substitutions to produce 2,4,6-tribromophenol

• white precipitate

• distinctly smells of antiseptic

• decolourises bromine water

benzene reaction with bromine water

• cannot react with bromine water

• due to increased reactivity of phenol because of the delocalised lone pair of electrons on the oxygen atom

• therefore electron density is greater

  • less stable

  • more susceptible to attack from electrophiles compared to benzene

combustion of benzene

• reacts with oxygen to produce CO2 and H2O

• produces a smoky flame due to the high C content of benzene

2C6H6 + 15O2 → 12CO2 + 6H2O

phenol + Br2 →

2,4,6-tribromophenol + HBr

white ppt formed

no catalyst

phenol + NaOH

- H+ as NaOH protonates

sodium phenate + H2O produced

phenol + CH3COCl

• forms an ester

• phenylethanoate + HCl

why are quaternary ammonium salts not amines?

do NOT possess a lone pair of electrons on the N

hydrogen bonding in amine molecules

weaker than in alcohols, though there is some

NOT in tertiary amines as there is no H to form any

are amines bronsted lowry proton acceptors or donators?

weak acceptors (bases)

can act in this way due to the lone pair on the N

draw a reaction between a secondary amine + water

order of base strength of amines

tertiary > secondary > primary > NH3 > aromatic amine

explain the order of base strength of amine

the higher the electron density of the lone pair in the N, the better it is able to accept the H+, so the base is stronger

primary, secondary and tertiary amines compared to ammonia

• the more alkyl groups that are substituted onto the N in place of H, the more electron density is pushed onto the N atom

• this is because of the inductive effect of alkyl groups compared to H atoms

• the more alkyl groups, the higher the electron density of the lone pair on the N

• hence it is a stronger base

ammonia compared to aromatic amines

• lone pair on the N is partially delocalised into the benzene ring

• leads to a reduction in the electron density on the N atom

• weaker base strength

primary aliphatic amine (butylamine) reaction with acid

forms an ammonium salt

amine acts as a base and accepts a proton to form a quaternary ammonium salt

primary aliphatic amines (butlyamine) reactions with ethanoyl chloride

N-subsitituted amide is produced

primary aliphatic amines (butlyamine) reactions with halogenoalkanes

H’s on the N are successively replaced by the R group from the haloalkanes

forms a secondary, tertiary then quaternary salt

primary aliphatic amines (butlyamine) reactions with copper (II) ions

copper ions react with the water to form a copper aqua ion

amine acts as a base to accept the protons from the water ligands

blue precipitate of Cu(OH)2(H2O)4 formed

ammonium salt formed

primary aliphatic amine (butylamine) reactions with ethanoyl chloride/acid anhydride

first produces an amide and a molecule of HCl/carboxylic acid

a second molecule of the amine reacts with the HCl/carboxylic acid to form a salt

via nucleophilic addition for acyl chlorides

what are the two ways of preparing a primary aliphatic amine

from a haloalkane

by the reduction of nitriles

producing a primary aliphatic amine from a haloalkane

• haloalkane reacted with an excess of conc. ammonia dissolved in ethanol at pressure in a sealed container

• via nucleophilic substitution

• not usually used as the amine can react further with the haloalkane to produce secondary and tertiary

  • problem minimised by using an excess of conc. ammonia

  • still a problem due to low yield

production of aliphatic primary amines by the reductions of nitriles

• only froms one amine rather than a mixture

• two steps

• adds a C to the chain

1. formation of a nirtile from a haloalkane

R-X + KCN → (warm ethanol) R - :C=-N + KX

2. reduction of the nirtile to form an amine

R-C=_N + 4[H] → (LiAlH4 in ethoxyethane) R-CH2-NH2

OR

R-C=_N + 2H2 → (Ni catalyst in heat) R-CH2-NH2

equations for the production of ethylamine

CH3-BR + KCN → CH3-C=_N + KBr

CH3-C=_N + 4[H] → CH3-CH2-NH2

two ways to prepare an amide

carboxylic acid + amine → amide + H2O

acyl chloride + amine → amide (or N-substituted amide) + HCl

how are polyamides formed

condensation polymerisation reaction

water is released

dicarboxylic acid + diamine

dicarboxylic acid + diol

polyester formed, water released

eg ethane-1,2-diol + benzene-1,4-dicarboxylic acid makes terylene/PET

draw the monomer structure of an amino acid

draw the polymer structure of an amino acid

draw the repeating unit of an amino acid

isoelectric point

the pH at which the amino acid becomes positively and negatively charged, so the net charge is zero

when a zwitterion forms

zwitterion in acidic conditions (pH below 7)

COO- group is more likely to accept a hydrogen ion, producing a positive end to the molecule

zwitterion in basic conditions

the hydrogen ion in the NH3 group is more likely to be lose, producing a negative end to the molecule

amino acid in neutral pH

is in a reversible reaction with a zwitterion, where the H on the OH joins the NH2 group to form NH3+

amino acid in acidic conditions

protonated

NH2 group becomes NH3

amino acid in alkaline conditions

deprotonated

any Hs bonded to a COO- group are lost

what makes an amino acid polar

the OH group on the COOH

how are aromatic amines produced

reduction of nitrobenzene using conc. HCl

tin catalyst

chirality of amino acids

all amino acids are chiral except glycine, whose R group is just H

they are therefore optically active, so a solution of amino acids rotates plane polarised monochromatic light

reactions of the NH2 group on the amino acid

• protonated by acids

• acylation w/acid chloride or acid anhydride

• nucleophilic substitution w/ haloalkanes

reactions of the COOH group of an amino acid

• deprotonated by bases

• esterification with alcohols and acid catalyst

chromatography

• proteins are a form of condensation polymer

• sequence of amino acids joined by peptide bonds -C=ONH-

• hydrolysed into constituent amino acids

• separated an identified by thin layer chromatography

• ninhydrin produces purple dots