Module 6: Organic chemistry and analysis

aromatic compounds

compounds that contain benzene ring

benzene is

• a major feedstock used in many industries: polymers, pharmaceuticals, dyes, explosives.

• benzene itself is highly carcinogenic

• simplest arene with an empirical formula of CH and a molecular formula of C₆H₆

kekule model

a cyclic structure for benzene with 3 alternating carbon-carbon double bonds

disproving kekule model

1. The lack of reactivity of benzene

2. The length of the carbon-carbon bonds in benzene

3. Hydrogenation Enthalpy of benzene

The lack of reactivity of benzene

If benzene had C=C double bonds it would decolour bromine water in an electrophilic addition reaction.

 But:

• Benzene does not undergo electrophilic addition reactions

• Benzene does not decolourise bromine under normal conditions, it requires a catalyst to react

This suggests there are no C=C double bonds in benzene.

bond length

• can use x ray diffraction to determine bond length

• prediction: C-C and C=C have different bond lengths so benzene would be an irregular hexagon if Kekulé’s model were true
  
  BUT

• benzene has a regular hexagon shape with equal bond lengths of 0.139nm which is in between the bond lengths of a C-C bond and a C-C bond
  

Hydrogenation enthalpy

 the enthalpy change when one mole of an unsaturated compound reacts with an excess of hydrogen to become a fully saturated compound

Hydrogenation enthalpy of benzene

• expected to have a enthalpy change of hydrogenation that is 3x the enthalpy change of hydrogenation of cyclohexene (-360kJmol-1)
  - cyclohexene has 1 double bond and the Kekulé structure of benzene has 3 double bond

But: 

The enthalpy change of hydrogenation of benzene is -208kJmol^-1 therefore the actual structure of benzene is more stable than expected. Less exothermic than expected.

bonding if kekule was correct

• 3 of the carbon electrons in sigma bonds between each carbon-carbon and each carbon-hydrogen

• remaining carbon electron in a p orbital adjacent to carbon atom resulting in alternating pi bonds between carbons where there is high electron density

Delocalised Model of Benzene

1. Each carbon creates three covalent σ-bonds with 3 of its electrons.

2. The remaining electron is found in a p-orbital at a right angle above and below the carbon atom.

3. the p-orbitals overlap evenly creating a ring of delocalised π-electrons above and below the structure.
   
   rings allow charge to be evenly spread across the molecule, making it stable and allowing equal bond lengths.

why doesnt benzene undergo electrophilic addition

Benzene has a lower electron density because the electrons are delocalised across the ring. This means it is less able to attract the electrophile and will be destabilised if groups are added to two neighbouring carbons

what reaction does benzene undergo?

electrophilic substitution

naming aromatic compounds- sufffix

• benzene ring is often considered to be the parent chain.

• Alkyl groups (e.g. -CH3, -C2H5), halogens (F, Cl, Br, I) and nitro groups are all prefixes to benzene.

naming aromatic compounds- suffix

• when benzene ring attached to alkyl chain with a functional group or to an alkyl chain with more than 7 carbons, benzene is considered the substituent.

• instead of benzene, the prefix phenyl is used in the name

naming aromatic compounds- exceptions

Benzoic acid (C6H5COOH)

Phenylamine (C6H5NH2)

Benzaldehyde (C6H5CHO)

Phenol (C6H5OH)

electrophile

electron pair acceptor

electrophilic substitution of benzene

• replacing a hydrogen for another group

• General equation: C6H6 + X+ → C6H5X + H+

  • X+ is the electrophile

mechanism of the reaction of an alkene and bromine

1. The π-bond in the alkene contains localised electrons above and below the C=C plane. This is an area of high electron density

2. The localised electrons induce a dipole in the non-polar bromine molecule making one bromine slightly positive and the other slightly negative

3. The slightly positive bromine atom enables the bromine to act as an electrophile

Comparing the Reactivity of Alkenes with Arenes

Because benzene has delocalised electrons above and below the plane of the carbon atoms in the ring structure. There is less electron density around around 2 carbon atoms in a benzene ring than round a C=C double bond in an alkene.

synthetic pathways- aromatic compounds

electrophilic substitution reactions of benzene

1. Nitration

2. Halogenation

3. Alkylation

4. Acylation

nitration of benzene

• Reagent: conc. nitric acid (HNO₃)

• Catalyst: conc. sulfuric acid (H₂SO₄)

• Conditions: 50-55 °C (if higher multiple substitutions occur)

• electrophile is the nitronium ion (NO₂⁺)
  - H₂SO₄ + HNO₃ → HSO₄^– + NO₂⁺ + H₂O

• regeneration of catalyst
  - H⁺ + HSO₄^- → H₂SO₄

halogenation of benzene

• only react if halogen carrier is present
  - e.g. AIX3, FeX3

• electrophile formation: Br₂ + FeBr₃ → FeBr₄^- + Br⁺

• catalyst reformation: H⁺ + FeBr₄^– → FeBr₃ + HBr

alkylation of benzene

It is a similar reaction mechanism as halogenation, using a halogen carrier (AlCl₃) as a catalyst.

Acylation of benzene

uses halogen carrier AICl3 as catalyst

phenols

• aromatic compounds that have hydroxyl group (-OH) attached directly to the ring

• hydroxyl always takes position number 1

• C6H5OH

phenols as weak acid

C6H5OH ⇌ C6H5O– + H+

e.g. Hydroxides: C6H5 + NaOH -> C6H5O-Na+ + H2O

p-orbital overlap into the ring weakens the O-H bond so phenol can donate H+ and form stable phenoxide ions

reactivity of phenol

Phenol is more acidic than alcohols but less acidic than carboxylic acids (can compare Ka)

• Ethanol does not react with either sodium hydroxide (strong base) or sodium carbonate (weak base)

• Phenol reacts with sodium hydroxide but not sodium carbonate

• Carboxylic acids react with both sodium hydroxide and sodium carbonate

bromination of phenol

• forms a white precipitate (orange — colourless)

• does not need a halogen carrier (e.g. AlBr₃)

• carried out at room temperature

nitration of phenol

• dilute nitric acid at room temperature

• mixture of 2-nitrophenol and 4-nitrophenol is formed

phenol more reactive than benzene?

The oxygen atom bonded to the benzene has two lone pairs of electrons in p-orbitals.

As the ring is electron deficient, one of the p-orbitals overlaps into the ring structure to become delocalised.

• Thus increasing the ring’s electron density, which attracts electrophiles more strongly than benzene

phenol has a higher electron density so can polarise more readily

do directing groups lesson

make flAshcards icba

what does a carbonyl contain?

C = O functional group

list the carbonyls

aldehyde, ketone and carboxylic acid

where is the carbonyl functional group found on an aldehyde?

at the end of carbon chain- CHO

where is the carbonyl functional group found on an ketone?

any carbon thats not the end- CO

whats the ending for aldehyde, ketone?

• -al

• -one

do carbonyls and alkenes reacts in the same way and why?

• no, because C=O is polar whereas C=C is not

• carbonyls react with nucleophiles – nucleophiles are attracted to the slight positive charge on the carbon.

nucleophiles

electron pair donor

oxidation of aldehydes form what and with what?

• carboxylic acids

• reflux with Cr2O7^2–/H^+ (i.e. K2Cr2O7/H2SO4)

how to write equation of oxidation of aldehydes e.g. butanal?

what does the oxidation of ketones form?

it can’t undergo oxidation reactions- has lack of reactivity

what’s the difference in the type of reaction that carbonyls and alkenes undergo?

• alkenes undergo electrophilic addition

• carbonyls undergo nucleophilic addition

reactions of carbonyl compounds with NaBH4

• NaBH4 acts as an reducing agent to reduce aldehyde and ketones into alcohols

• warmed with the reducing agent in aqueous solution

reaction of aldehyde with NaBH4 e.g. butanal

• reduced to primary alcohols

reaction of ketones with NaBH4 e.g. propanone

• reduced to secondary alcohols

reactions of carbonyl compounds with HCN

• form hydroxynitriles

• using e.g. NaCN/H2SO4

• increases the length of the carbon chain

reaction of aldehyde with HCN e.g. propanal

• contains 2 functional groups

  • hydroxyl group (-OH)

  • nitrile group (C≡N)

mechanism for reaction between carbonyls and NaBH4

mechanism for reaction between carbonyls and NaCN/H^+

Testing for aldehydes and ketones

• Add 2,4- DNP

  • If an aldehyde or ketone is present an orange precipitate will form.

identifying aldehydes and ketones

• Using melting point- add 2,4 –DNP

  • orange precipitate forms

  • separate the solid from the solution

  • recrystallise the solid to form pure sample

  • the melting point is measured

  • compare to a database of known values

distinguishing between aldehydes and ketones

• Add Tollens reagent (silver nitrate in aqueous ammonia) in a water bath at 50 degrees celsius

• Silver mirror produced if an aldehyde is present

equation for aldehyde and tollens reagent

why can’t a ketone form a silver mirror with tollens reagent?

Silver ions are reduced to silver oxidising the aldehyde to a carboxylic acid. A ketone cannot be oxidised further.

carboxylic acid functional group

• carbonyl group

• hydroxyl group

solubility of carboxylic acids

• the C=O and O-H bonds in carboxylic acid are polar

• this allows carboxylic acids to form hydrogen bonds with water

• making it soluble

diagram of why carboxylic acids are soluble

what happens to the solubility of carboxylic acids as chain length increases?

Solubility decreases as chain length increases because the carbon chain is non-polar and cannot form hydrogen bonds with water

strength of carboxylic acids

HCOOH (aq) ⇌ H^+ (aq) + HCOO^- (aq)

• weak acids as they only partially dissociate in water

acid reactions of carboxylic acids

• redox reactions with metals

• neutralisation reactions with bases (alkalis, metal oxides, and carbonates)

carboxylic acids form carboxylate salts

carboxylate ions

Redox reactions of carboxylic acids + metal

• Carboxylic acid (aq) + metal (s) 🡪 Carboxylate salt (aq) + hydrogen (g)

• observations

  • Effervescence as hydrogen gas is evolved.

  • Metal disappearing as insoluble Mg reacts and forms the soluble salt

carboxylic acid + metal oxides

Carboxylic acids + metal oxide 🡪 carboxylate salt + water

carboxylic acid + metal hydroxides

Carboxylic acids + metal hydroxide 🡪 carboxylate salt + water

carboxylic acid + metal carbonate

Carboxylic acids + metal carbonate 🡪 salt + water + carbon dioxide

testing for carboxylic acids/carboxyl group

• the neutralisation reactions between carboxylic acids and carbonates (e.g sodium carbonate) are important

• carboxylic acids are the only common organic compounds sufficiently acidic to react with carbonates

• Observation: FIZZING (not gas is formed)

derivatives of carboxylic acids

• can be hydrolysed to form carboxylic acids

• have a common sequence of atoms in their structureknown as acyl group

examples of carboxylic acids

Esters, Acyl Chlorides, Acid anhydrides

esters structure

esters naming

1. remove the -oic acid suffix from parent carboxylic acid and replace with -oate

2. alkyl chain attached to oxygen atom of the COO group is then added as the first word in name

acyl chlorides structure

acyl chlorides naming

1. remove -oic acid suffix from parent carboxylic acid and replace with -oyl chloride

Acid anhydrides structure

Acid anhydrides naming

Acid anhydrides formation

• removal of water from 2 carboxylic acids

esterification

• reaction of an alcohol with a carboxylic acid to form an ester

• warmed with a carboxylic acid with small amount of conc. sulfuric acid (acts as a catalyst)

hydrolysis of esters

can be hydrolysed by either an aqueous acid or alkali

acid hydrolysis of esters

• reverse of esterification- forms carboxylic acid and alcohol

• ester heated under reflux with dilute aqueous acid

• ester broken down by water, with acid acting as a catalyst

alkaline hydrolysis of esters

• ester heated under reflux with aqueous hydroxide ions

• forms carboxylate ion and an alcohol

formation of acyl chloride

when SOCl2 reacts with a carboxylic acid
- other products SO2 and HCl are evolved as gases leaving just the acyl chloride

reactions of acyl chlorides

• very reactive

• react with nucleophiles by losing chloride ions whilst retaining C=O

Acyl chloride + Alcohol

• forms ester and HCl

Acyl chloride + Phenol

Acyl chloride + water

ammonia formula

NH3

ammonium structure

NH4^+

amine structures e.g. primary, secondary, tertiary

amine structures e.g. primary, secondary, tertiary

acyl chloride + ammonia

ammonia acts as a nucleophile

acyl chloride + primary amine

amines

• Derived from ammonia

• Replace hydrogen/s with an organic group

• amines fall into different classes depending on how many of the hydrogen atoms are replaced

NH2 group is on the end of a chain:

Suffix: Amine

Prefix: alkyl chain

NH2 group is middle of the chain

Suffix: Alkane chain

Prefix: Amino

more than 1 group attached to the Nitrogen

• Longest chain in front of amine

• N in front of every other chain coming off N

• Put in alphabetical order

how do amines act as bases

• ability to accept a proton (H+)

• have a lone pair of electrons which can accept a proton to form a dative covalent bond

• able to neutralise acids to make salts

preparation of amines-primary

• can act as a nucleophile in a nucleophilic substitution reaction with a haloalkane

• Conditions: excess ammonia (prevents further substitution of the amine), ethanol (solvent)

preparation of amines- secondary and tertiary

The product CH₃CH₂NH₂ still contains a lone pair of electrons on nitrogen that can react further with a haloalkane to form a secondary amine.

RCl + RNH₂ 🡪 R₂NH₂⁺Cl^-
- e.g. CH₃CH₂Cl + CH₃CH₂NH₂ 🡪 (CH₃CH₂)₂NH₂⁺Cl^-R₂NH₂⁺Cl^- + NaOH 🡪 R₂NH +NaCl + H₂O
- e.g. (CH₃CH₂)₂NH₂⁺Cl^- + NaOH 🡪 (CH₃CH₂)₂NH +NaCl + H₂O

• Tertiary amines can then be produced by a further reaction of a secondary amine with a haloalkane.

preparation of aromatic amines

• phenylamine by reduction of nitrobenzene

• nitrobenzene heated under reflux with tin and hydrochloric acid

• then reacted with excess sodium hydroxide

• tin and hydrochloric acid act as a reducing agent.

amino acids

• contain both an amine group (-NH2) and a carboxylic acid (-COOH) functional groups

• α-amino acids contain both an amine and a carboxyl group that are separated by one carbon atom

• general formula: RCH(NH2)COOH