Characteristic organic reactions - Chemistry (9701) - Cambridge A-Level

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Welcome to Characteristic Organic Reactions!

Hello future chemist! This chapter is the heart of A Level Organic Chemistry. Instead of memorizing long lists of molecules, we focus on functional groups—small parts of molecules that behave predictably. Understanding these groups and their characteristic reactions is like learning a language: once you know the verbs (reactions) and nouns (functional groups), you can predict what any new molecule will do!
Don't worry if the sheer number of reactions seems overwhelming. We will group them by mechanism and functional group to make them easy to digest. Let's get started!

1. The Language of Organic Reactions

1.1 Understanding Bond Fission

When a covalent bond breaks, there are two ways the electrons can split:

Homolytic Fission: Each atom gets one electron from the shared pair.

$A:B \rightarrow A\bullet + B\bullet$
This creates highly reactive species called Free Radicals, which have an unpaired electron. These reactions are often started by UV light (sunlight).

Heterolytic Fission: One atom gets both electrons.

$A:B \rightarrow A^+ + B:^-$
This creates charged ions: a Cation ($A^+$) and an Anion ($B:^-$). Most ionic/polar reactions follow this path.

Memory Aid: "Homo" means same (each gets one); "Hetero" means different (one gets both).

1.2 Identifying Reactive Species

Reactive species are molecules or ions that are searching for electrons or giving them away to achieve stability.

Nucleophile (Nu): An electron-rich species that 'loves' positive nuclei (nucleo = nucleus, phile = loving). They have a lone pair of electrons or a negative charge. They attack positive or partially positive centers.
Examples: $OH^-$, $CN^-$, $NH_3$, $H_2O$.
Electrophile (E): An electron-deficient species that 'loves' electrons. They have a positive charge or a partial positive charge. They attack electron-rich centers (like double bonds).
Example: $H^+$, $Br^+$, $NO_2^+$.

1.3 Main Types of Reactions

Understanding the reaction type helps you predict the products:

Addition: Two molecules combine to form one product. Typically occurs across double/triple bonds (e.g., alkenes).
Substitution: One atom or group replaces another atom or group (e.g., halogenoalkanes reacting with $OH^-$).
Elimination: A small molecule (like $H_2O$ or $HX$) is removed from a larger molecule, often forming a double bond.
Hydrolysis: Breaking a bond using water or dilute acid/alkali (e.g., esters or amides).
Condensation: Two molecules join together, eliminating a small molecule (usually $H_2O$), e.g., ester formation.
Oxidation and Reduction: In organic chemistry, oxidation often means adding oxygen or removing hydrogen; reduction means adding hydrogen or removing oxygen. We often use $[O]$ or $[H]$ in simplified equations.

Key Takeaway: Organic reactions are classified by how the bonds break (fission) and what type of reactive species is involved (nucleophile/electrophile).

2. Alkanes: Free-Radical Substitution (FRS)

Alkanes are saturated hydrocarbons (only single C-C bonds). They are generally unreactive because:
1. The $C-H$ and $C-C$ bonds are strong and require a lot of energy to break.
2. The $C-H$ bonds have very low polarity, so they don't attract polar reagents (like nucleophiles or electrophiles).

2.1 Halogenation: The Free-Radical Substitution Mechanism

Alkanes react with halogens ($Cl_2$ or $Br_2$) only in the presence of UV light or high heat. This is a substitution reaction where an H atom is replaced by a halogen atom.

Reagents & Conditions: $Cl_2$ or $Br_2$ and UV light (or heat).

Step-by-Step Mechanism: FRS (e.g., Methane with Chlorine)

Initiation: The UV light provides energy for homolytic fission of the weak halogen bond, creating two free radicals.
$Cl-Cl \xrightarrow{UV\ light} 2Cl\bullet$
Propagation: This is the chain reaction stage where radicals react to form new radicals.
Step 1: $Cl\bullet$ attacks the alkane, producing an alkyl radical and $HCl$.
$CH_4 + Cl\bullet \rightarrow \bullet CH_3 + HCl$
Step 2: The alkyl radical attacks a $Cl_2$ molecule, forming the product and regenerating the original radical type ($Cl\bullet$).
$\bullet CH_3 + Cl_2 \rightarrow CH_3Cl + Cl\bullet$
Termination: Any two radicals combine, ending the chain reaction and producing a stable molecule. This often leads to mixtures of products.
$Cl\bullet + Cl\bullet \rightarrow Cl_2$
$\bullet CH_3 + Cl\bullet \rightarrow CH_3Cl$
$\bullet CH_3 + \bullet CH_3 \rightarrow CH_3CH_3$

Common Mistake: Forgetting that FRS often produces a mixture of products (e.g., mono-, di-, tri-substituted products, and longer chain alkanes from termination).

Key Takeaway: Alkanes only react via Free-Radical Substitution, initiated by UV light, due to their strong, non-polar bonds.

3. Alkenes: Electrophilic Addition

Alkenes are unsaturated hydrocarbons containing a $C=C$ double bond. The double bond consists of one strong $\sigma$ bond and one weak $\pi$ bond. The $\pi$ bond has high electron density and is easily broken, making alkenes highly reactive, especially towards Electrophiles.

3.1 Electrophilic Addition Mechanism (EAS)

The mechanism involves an electrophile attacking the electron-rich double bond, leading to the formation of a single product.

The General Steps:

The electron pair from the $C=C$ $\pi$ bond attacks the electrophile ($E^+$), forming a carbocation (a species where a C atom only has 3 bonds and a positive charge).
The negatively charged species (nucleophile, $Nu^-$) rapidly attacks the positively charged carbocation.

3.2 Key Addition Reactions and Reagents

Addition of Hydrogen (Hydrogenation): Forms an alkane.
Reagents/Conditions: $H_2(g)$ and Pt/Ni catalyst, heat.
Addition of Halogens (Halogenation): Forms a halogenoalkane (e.g., $Br_2$ turns brown solution colorless). Used as a test for unsaturation.
Reagents/Conditions: $X_2$ (e.g., $Br_2$) in an inert solvent like $CCl_4$ or aqueous bromine, room temperature.
Addition of Hydrogen Halides (HX): Forms a halogenoalkane.
Reagents/Conditions: $HX(g)$ at room temperature.
Addition of Steam (Hydration): Forms an alcohol.
Reagents/Conditions: $H_2O(g)$ (steam) and $H_3PO_4$ catalyst, heat/high pressure.
Oxidation (Mild): Forms a diol (a molecule with two -OH groups).
Reagents/Conditions: Cold, dilute, acidified $KMnO_4$. (Colour change: Purple to colourless/brown precipitate).
Oxidation (Vigorous): Causes the rupture (breaking) of the $C=C$ bond, leading to ketones, carboxylic acids, or $CO_2$. This is used to determine the position of the original double bond.
Reagents/Conditions: Hot, concentrated, acidified $KMnO_4$.

3.3 Markovnikov's Rule and Carbocation Stability

When adding an asymmetrical reagent (like $HX$ or $H_2O$) to an asymmetrical alkene (like propene), two products are possible. Markovnikov's rule helps predict the major product.

The Rule (simplified): The hydrogen atom of the reagent adds to the carbon atom of the double bond that already has more hydrogen atoms.

The Chemical Explanation: This occurs because the reaction proceeds via the most stable intermediate carbocation. Alkyl groups ($R$) have an inductive effect, donating electron density, which helps stabilize the positive charge on the carbocation.

Stability Order: Tertiary ($3^\circ$) > Secondary ($2^\circ$) > Primary ($1^\circ$)

Analogy: Imagine the carbocation is sick. The more alkyl groups surrounding it (3rd degree), the more "friends" it has to share the burden of the positive charge, making it more stable.

Key Takeaway: Alkenes undergo Electrophilic Addition due to the weak $\pi$ bond. Markovnikov's rule is explained by the stability order of carbocation intermediates ($3^\circ > 2^\circ > 1^\circ$).

4. Halogenoalkanes: Nucleophilic Substitution ($S_N$) and Elimination

Halogenoalkanes contain a $C-X$ bond ($X = Cl, Br, I$). Because the halogen is much more electronegative than carbon, the bond is polar ($\delta^+C-X\delta^-$). This makes the carbon atom susceptible to attack by nucleophiles.

4.1 Nucleophilic Substitution Reactions

The halogen ($X$) is replaced by a nucleophile ($Nu^-$). The $C-X$ bond is broken, and a new $C-Nu$ bond is formed.

Reagent/Conditions	Nucleophile	Product
$NaOH(aq)$, heat	$OH^-$	Alcohol (Hydrolysis)
$KCN$ in ethanol, heat	$CN^-$	Nitrile (Chain lengthened by one C atom)
$NH_3$ in ethanol, sealed tube/pressure	$NH_3$	Amine (Primary)

4.2 Elimination Reaction (Forming Alkenes)

Halogenoalkanes can also undergo elimination to form an alkene.

Reagents & Conditions: $NaOH$ in ethanol, heat. (Note: Using aqueous $NaOH$ leads to substitution; using ethanolic $NaOH$ leads to elimination.)

4.3 Mechanism: $S_N1$ vs. $S_N2$

The mechanism depends on the structure of the halogenoalkane ($1^\circ$, $2^\circ$, or $3^\circ$).

$S_N2$ (Substitution Nucleophilic Bimolecular):
This is a one-step mechanism. The nucleophile attacks the C atom simultaneously as the $C-X$ bond breaks.
Preference: Primary ($1^\circ$) halogenoalkanes (least steric hindrance).
$S_N1$ (Substitution Nucleophilic Unimolecular):
This is a two-step mechanism.
1. Slow step (Rate Determining): $C-X$ bond breaks, forming a planar carbocation ($3^\circ$ carbocations are highly stable due to inductive effect).
2. Fast step: The nucleophile attacks the carbocation.
Preference: Tertiary ($3^\circ$) halogenoalkanes (stability of the $3^\circ$ carbocation).

4.4 Relative Reactivity

The reactivity of halogenoalkanes in substitution depends primarily on the strength of the $C-X$ bond. Weaker bonds break more easily.

Bond Strength Order: $C-Cl > C-Br > C-I$
Reactivity Order: Iodoalkanes > Bromoalkanes > Chloroalkanes

This difference is demonstrated by the reaction with aqueous silver nitrate in ethanol:
The rate at which a precipitate forms indicates the bond strength ($C-I$ forms yellow $AgI$ fastest; $C-Cl$ forms white $AgCl$ slowest).

Quick Review Box: Halogenoalkanes

$1^\circ$: Mostly $S_N2$, Elimination only with ethanolic alkali.
$3^\circ$: Mostly $S_N1$, faster reaction due to stable carbocation intermediate.

5. Alcohols and Phenol: Oxidation and Acidity

5.1 Alcohols (R-OH)

Alcohols are classified by the number of carbons attached to the carbon bearing the -OH group ($1^\circ$, $2^\circ$, $3^\circ$).

A. Reactions to Make Alcohols (Synthesis)

From Alkenes: Electrophilic addition of steam ($H_2O(g)$ and $H_3PO_4$ catalyst).
From Halogenoalkanes: Nucleophilic substitution using $NaOH(aq)$ and heat.
Reduction of Carbonyls/Carboxylic Acids: Reduction using powerful reducing agents like $LiAlH_4$ (or $NaBH_4$ for just carbonyls).
Hydrolysis of Esters: Using dilute acid or dilute alkali and heat.

B. Characteristic Reactions of Alcohols

Reaction with Na metal: Forms hydrogen gas and an alkoxide salt.
$2ROH + 2Na \rightarrow 2RO^-Na^+ + H_2(g)$
Substitution (Halogenation): Forms halogenoalkanes.
Reagents: $HX(g)$ (e.g., $HBr$), or $PCl_3/$heat, $PCl_5$, or $SOCl_2$.
Dehydration (Elimination): Forms an alkene (the reverse of hydration).
Reagents: Heated catalyst (e.g., $Al_2O_3$) or concentrated $H_2SO_4$.
Esterification: Forms an ester (condensation reaction) when reacting with a carboxylic acid.
Reagents: Carboxylic acid, concentrated $H_2SO_4$ catalyst.
Did You Know? This reaction is slow and reversible, hence the need for a strong acid catalyst.

C. Oxidation Reactions (The Key Difference Maker)

Oxidation is the most important reaction for distinguishing between alcohol classes. Reagents: Acidified potassium dichromate(VI) ($K_2Cr_2O_7$) (Orange to Green) or acidified potassium manganate(VII) ($KMnO_4$) (Purple to Colourless).

Primary ($1^\circ$) Alcohols:
- To Aldehyde: Use distillation to immediately remove the aldehyde product (preventing further oxidation).
- To Carboxylic Acid: Use refluxing (heat for a sustained period) to ensure complete oxidation.
Secondary ($2^\circ$) Alcohols: Oxidised to Ketones (Ketones resist further mild oxidation).
Tertiary ($3^\circ$) Alcohols: Cannot be oxidised under these mild conditions.

D. The Iodoform Test (Tri-iodomethane Test)

This test detects the presence of the $CH_3CH(OH)-$ group (or the $CH_3CO-$ group in carbonyls).
Reagents: Alkaline $I_2(aq)$ (or $I_2$ and $NaOH(aq)$).
Positive result: Formation of a yellow precipitate of tri-iodomethane ($CHI_3$).

5.2 Phenol (Benzene with -OH)

Acidity Comparison

Phenol is much more acidic than ethanol but less acidic than carboxylic acids.

Acidity Order: Carboxylic Acid > Phenol > Water > Ethanol

Explanation: Phenol's acidity is due to the stabilisation of the phenoxide ion ($C_6H_5O^-$) by delocalisation of the negative charge into the benzene ring. Ethanol lacks this stabilisation.

Reactions of Phenol

The -OH group is a powerful electron-donating group, making the benzene ring highly reactive towards electrophilic substitution, especially at the 2-, 4-, and 6-positions.

With Bases: Reacts with $NaOH(aq)$ (but not carbonates, unlike carboxylic acids) to form sodium phenoxide.
Nitration: Requires only dilute $HNO_3(aq)$ at room temperature (compare to benzene, which needs concentrated acids and heat) to form 2- and 4-nitrophenol.
Bromination: Reacts instantly with bromine water ($Br_2(aq)$), turning it colourless and forming a white precipitate of 2,4,6-tribromophenol.

Key Takeaway: Oxidation is crucial for classifying alcohols. Phenol is more acidic than water and ethanol due to charge stabilization, making its ring highly susceptible to electrophilic attack.

6. Carbonyl Compounds: Aldehydes and Ketones

Carbonyl compounds contain the $C=O$ functional group. The oxygen atom is highly electronegative, creating a strong dipole ($\delta^+C=O\delta^-$). This means the carbon atom is electron deficient and attacked by Nucleophiles.

6.1 Nucleophilic Addition Mechanism

The primary reaction type is Nucleophilic Addition. The nucleophile attacks the $\delta^+$ carbon, opening up the $\pi$ bond.

A. Reduction to Alcohols

Aldehydes and ketones are reduced back into alcohols.

Aldehydes reduce to primary ($1^\circ$) alcohols.
Ketones reduce to secondary ($2^\circ$) alcohols.

Reagents:

$NaBH_4$

$LiAlH_4$

B. Formation of Hydroxynitriles

Reaction with Hydrogen Cyanide forms a hydroxynitrile, increasing the chain length by one carbon atom.

Reagents/Conditions: $HCN$ (hydrogen cyanide), with a catalyst like $KCN$, and heat.

Mechanism: The reaction is catalysed by $CN^-$ ions. The nucleophilic $CN^-$ attacks the carbonyl carbon, followed by protonation from $HCN$ or water.

6.2 Chemical Tests (Detection and Distinction)

2,4-DNPH (2,4-Dinitrophenylhydrazine) Test: (Detection)
This test detects the presence of any carbonyl group (aldehyde or ketone).
Result: Formation of a yellow, orange, or red precipitate (hydrazone derivative).
Fehling's/Tollens' Test: (Distinction: Aldehyde vs. Ketone)
Aldehydes are easily oxidised to carboxylic acids, while ketones are not (under mild conditions).
- Tollens' Reagent (Ammoniacal Silver Nitrate):
  Aldehyde: Forms a silver mirror on the test tube.
  Ketone: No reaction.
- Fehling's Solution (Alkaline Copper(II) Ions):
  Aldehyde: Blue solution turns into a red/brown precipitate of copper(I) oxide.
  Ketone: No reaction.
Tri-iodomethane Test: (Detection of $CH_3CO-$ group)
This test detects the presence of the methyl carbonyl group ($CH_3CO-R$, where $R$ can be $H$ or an alkyl group).
Reagents: Alkaline $I_2(aq)$.
Result: Formation of a yellow precipitate ($CHI_3$). (Ethanal and propanone give positive results).

Key Takeaway: Carbonyls undergo Nucleophilic Addition due to the polar $C=O$ bond. Aldehydes are distinct because they are easily oxidised, unlike ketones.

7. Carboxylic Acids, Esters, and Acyl Chlorides

7.1 Carboxylic Acids (RCOOH)

Carboxylic acids are weak acids, dissociating slightly in water. They are significantly stronger acids than alcohols and phenols because the carboxylate ion ($RCOO^-$) is highly stabilised by resonance (delocalisation of charge across both oxygen atoms).

A. Synthesis of Carboxylic Acids

Oxidation: Refluxing a $1^\circ$ alcohol or aldehyde with acidified $K_2Cr_2O_7$ or $KMnO_4$.
Hydrolysis of Nitriles: Heating a nitrile with dilute acid or dilute alkali, followed by acidification.
Hydrolysis of Esters: Heating an ester with dilute acid or dilute alkali, followed by acidification.

B. Reactions of Carboxylic Acids (Acidic Behaviour)

Reaction with Reactive Metals: Forms a salt and $H_2(g)$.
Example: $2RCOOH + 2Na \rightarrow 2RCOO^-Na^+ + H_2$
Reaction with Alkalis (Neutralisation): Forms a salt and $H_2O$.
Example: $RCOOH + NaOH \rightarrow RCOO^-Na^+ + H_2O$
Reaction with Carbonates/Bicarbonates: Forms a salt, $H_2O$, and $CO_2(g)$. This is a common test to distinguish carboxylic acids from phenols.

C. Other Reactions

Reduction: Reduced to a $1^\circ$ alcohol using the strong reducing agent $LiAlH_4$ (Lithium Aluminum Hydride).
Esterification: Forms an ester when reacting with an alcohol, catalysed by concentrated $H_2SO_4$.
Formation of Acyl Chlorides: Replacement of the -OH group by -Cl using $PCl_3$, $PCl_5$, or $SOCl_2$ (thionyl chloride). This is how we make the highly reactive derivative.

Further Oxidation: Methanoic acid ($HCOOH$) and ethanedioic acid ($HOOCCOOH$) are unique as they contain a $C-H$ bond adjacent to the carboxyl group, allowing them to be oxidised further (e.g., $HCOOH$ $\rightarrow$ $CO_2$ and $H_2O$, detectable using Tollens' or Fehling's).

7.2 Esters (RCOOR')

Esters are derived from carboxylic acids and alcohols. They smell nice!

Preparation: Condensation between a carboxylic acid and an alcohol (Acid catalyst, $H_2SO_4$) OR reaction between an alcohol/phenol and an acyl chloride (see below).
Hydrolysis: Breaking the ester linkage.
- Acid Hydrolysis (reflux with dilute acid): Forms the original carboxylic acid and alcohol (reversible).
- Alkaline Hydrolysis (saponification, reflux with dilute alkali): Forms the carboxylate salt and alcohol (irreversible, used to make soap).

7.3 Acyl Chlorides (RCOCl)

Acyl chlorides are the most reactive carboxylic acid derivatives due to the strong inductive effect of the chlorine atom and the $C=O$ group, making the carbonyl carbon highly $\delta^+$.

They react via an Addition-Elimination mechanism (sometimes called Nucleophilic Acyl Substitution) very readily, even at room temperature.

Reagent	Product	Side Product
$H_2O$ (Hydrolysis)	Carboxylic Acid	$HCl$
Alcohol ($R'OH$)	Ester	$HCl$
Ammonia ($NH_3$)	Amide (Primary)	$HCl$
Primary Amine ($R'NH_2$)	Amide (Secondary)	$HCl$

Reactivity Reminder: Acyl chlorides are hydrolysed much faster than alkyl chlorides (halogenoalkanes), which are much faster than aryl chlorides (halogenoarenes).

Key Takeaway: Carboxylic acids are weak acids stabilised by resonance. Acyl chlorides are highly reactive and react via Addition-Elimination to form esters and amides easily.

8. Nitrogen Compounds: Amines, Nitriles, Amides, and Amino Acids

8.1 Primary Amines ($RNH_2$ or $ArNH_2$)

A. Basicity of Amines

Amines are Brønsted-Lowry bases because the nitrogen atom has a lone pair of electrons that can accept a proton ($H^+$).

Basicity Order (in aqueous solution): Ethylamine > Ammonia > Phenylamine

Alkyl Amines (Ethylamine): The alkyl group ($CH_3CH_2-$) has an inductive effect, pushing electron density onto the nitrogen, making the lone pair more available for $H^+$ attack. (Stronger base).
Ammonia ($NH_3$): Acts as a baseline base.
Phenylamine ($C_6H_5NH_2$): The lone pair on the N atom is delocalised into the benzene ring, making it less available for accepting a proton. (Weaker base).

B. Synthesis of Amines

The synthesis often involves reduction:

From Halogenoalkanes: Reaction with $NH_3$ in ethanol, heated under pressure. (Note: difficult to control, often yields secondary and tertiary amines too).
From Amides: Reduction of the carbonyl group ($C=O$) using $LiAlH_4$.
From Nitriles: Reduction of the nitrile group ($-C\equiv N$) using $LiAlH_4$ or $H_2/Ni$.
Phenylamine (Aromatic Amine): Preparation from nitrobenzene (via nitration of benzene), followed by reduction using hot $Sn$/conc. $HCl$ and then $NaOH(aq)$.

8.2 Amides (RCONH$R'$)

Amides are formed from the reaction of acyl chlorides with ammonia or primary/secondary amines.

Hydrolysis: Like esters, amides can be hydrolysed by heating with aqueous acid or aqueous alkali to yield a carboxylic acid (or carboxylate salt) and an amine (or ammonium salt).
Reduction: Reduced by $LiAlH_4$ to form an amine.

Amides are much weaker bases than amines because the nitrogen's lone pair is partly delocalised towards the adjacent carbonyl oxygen.

8.3 Amino Acids

These molecules contain both an acidic carboxyl group (-COOH) and a basic amine group ($-\text{NH}_2$).

Zwitterions: In solution, the acid protonates the amine, forming a dipolar ion called a zwitterion ($H_3N^+CHRCOO^-$). The $pH$ at which the molecule exists primarily as a zwitterion is the isoelectric point.
Peptide Bonds: Amino acids join together in a condensation reaction between the carboxyl group of one molecule and the amine group of another, forming an amide bond, known as a peptide bond in biological context.

8.4 Azo Compounds (A Level Only)

Azo compounds ($R-N=N-R'$) are commonly used as dyes because they absorb light strongly.

Key Reaction (Diazotisation and Coupling):

Phenylamine reacts with nitrous acid ($HNO_2$) (generated in situ from $NaNO_2$ and dilute acid) below $10^\circ C$ to form a benzenediazonium salt ($ArN_2^+$).
This diazonium salt then reacts (couples) with an activated aromatic molecule (like phenol in $NaOH(aq)$) to form the colourful azo compound, containing the $-N=N-$ azo group.

Key Takeaway: Amines are bases due to the lone pair, but basicity is affected by electron-donating alkyl groups (increasing it) or delocalisation into a ring (decreasing it). Amino acids form zwitterions and link via peptide (amide) bonds.

9. Arenes (Aromatic Compounds): Electrophilic Substitution (EAS)

Arenes, like benzene, are very stable due to the delocalised $\pi$ electron system. Unlike alkenes, they do not undergo addition reactions because that would destroy the stability of the aromatic ring. Instead, they react via Electrophilic Aromatic Substitution (EAS).

9.1 General Mechanism: Electrophilic Aromatic Substitution

A strong electrophile ($E^+$) is required, often generated by a Lewis acid catalyst (like $AlCl_3$).

The electrophile attacks the delocalised $\pi$ system, forming a highly unstable, positively charged intermediate (a cyclohexadienyl cation). This step breaks the aromaticity and is slow.
A proton ($H^+$) is removed from the intermediate by a base, regenerating the delocalised ring structure and thus restoring aromatic stability. This step is fast.

9.2 Key Reactions of Benzene

All these reactions require heat and a strong catalyst to generate the necessary powerful electrophile.

Reaction	Reagents/Conditions	Electrophile	Product
Nitration	Conc. $HNO_3$ and Conc. $H_2SO_4$, $25-60^\circ C$	$NO_2^+$ (Nitronium ion)	Nitrobenzene
Halogenation	$Cl_2$ or $Br_2$ and catalyst ($AlCl_3$ or $AlBr_3$)	$X^+$ (polarised halogen)	Halogenoarene
Friedel-Crafts Alkylation	$CH_3Cl$ and $AlCl_3$ (catalyst), heat	$CH_3^+$	Methylbenzene (Toluene)
Friedel-Crafts Acylation	$CH_3COCl$ (acyl chloride) and $AlCl_3$, heat	$CH_3CO^+$	Phenylethanone
Hydrogenation (Addition)	$H_2$, Pt/Ni catalyst, heat, high pressure	N/A (Special case)	Cyclohexane (Destroys aromaticity)

9.3 Reactions of Alkylbenzenes (Side Chain)

If the arene has an alkyl side chain (e.g., methylbenzene), the ring remains stable, but the side chain can react under different conditions:

Side-chain Halogenation: Reaction with $Cl_2$ or $Br_2$ in the presence of UV light (Free-Radical Substitution). This is the same reaction as alkanes.
Complete Side-chain Oxidation: Oxidation using hot alkaline $KMnO_4$ followed by dilute acid converts the entire side chain (no matter how long) into a carboxylic acid group, producing benzoic acid.

9.4 Directing Effects of Substituents

When a second substitution occurs on a monosubstituted benzene ring, the position of the second substituent is determined by the first group already attached.

2, 4, 6-directors (ortho/para directors): These groups activate the ring (make it more reactive) by pushing electrons into the ring.
Examples: $-NH_2$, $-OH$, $-R$ (alkyl group).
3-directors (meta directors): These groups deactivate the ring (make it less reactive) by pulling electrons out of the ring.
Examples: $-NO_2$, $-COOH$, $-COR$.

9.5 Halogenoarenes vs. Halogenoalkanes

Halogenoarenes (e.g., chlorobenzene) are much less reactive towards nucleophilic substitution than halogenoalkanes (e.g., chloroethane).

Explanation: In halogenoarenes, the lone pair on the halogen is partially delocalised into the benzene ring. This makes the $C-X$ bond shorter and stronger, requiring significantly more energy to break, hence substitution is difficult.

Key Takeaway: Benzene’s stability forces it to undergo Electrophilic Substitution (EAS) using strong electrophiles generated by catalysts. Substituents control where the next group attacks (directing effects).

10. Polymerisation: Making Giant Molecules

Polymerisation is the process of linking many small molecules (monomers) together to form a large chain molecule (polymer).

10.1 Addition Polymerisation

Occurs when monomers contain $C=C$ double bonds (usually alkenes). The $\pi$ bond breaks and single bonds form between the monomers, resulting in a single polymer product.

Monomer: ethene ($C_2H_4$) $\rightarrow$ Polymer: poly(ethene) (or polythene)
Monomer: chloroethene (vinyl chloride) $\rightarrow$ Polymer: poly(chloroethene) (or PVC)

The Repeat Unit: You deduce the repeat unit by opening the double bond of the monomer.

10.2 Condensation Polymerisation (A Level Only)

Condensation polymers form when two different monomers (or one monomer with two different functional groups) react, and a small molecule (usually $H_2O$ or $HCl$) is eliminated. The monomers must be difunctional (e.g., diol and dicarboxylic acid).

A. Polyesters (Ester linkage)

Formed from the reaction between:
1. A diol (two -OH groups) and a dicarboxylic acid (two -COOH groups).
2. A diol and a dioyl chloride (more reactive).
3. A single hydroxycarboxylic acid (contains both -OH and -COOH).

B. Polyamides (Amide/Peptide linkage)

Formed from the reaction between:
1. A diamine (two $-NH_2$ groups) and a dicarboxylic acid.
2. A diamine and a dioyl chloride (more reactive).
3. A single aminocarboxylic acid (contains both $-NH_2$ and -COOH, like amino acids).

10.3 Degradable Polymers

The disposal of polymers is an environmental concern.

Poly(alkenes) (Addition Polymers): Chemically inert (unreactive) because they only contain strong $C-C$ and $C-H$ bonds. This makes them non-biodegradable. They also produce harmful combustion products when incinerated.
Polyesters and Polyamides (Condensation Polymers): Are biodegradable because they contain functional groups (ester/amide linkages) that can be easily broken down by acidic or alkaline hydrolysis.
Some polymers can also be degraded by the action of light (photodegradable).

Key Takeaway: Addition polymers (like poly(ethene)) are inert and difficult to dispose of. Condensation polymers (Polyesters/Polyamides) contain linkages that make them biodegradable via hydrolysis.

11. Organic Synthesis: Putting the Steps Together

Organic synthesis involves planning multi-step routes to prepare a target molecule from a starting material. This requires you to work backwards and forwards through the reaction types covered in all chapters.

Strategy Tips:

Identify the Change: What functional group needs to be created or converted? Has the carbon chain length changed?
Work Backwards (Retrosynthesis): Look at the final product. What was the immediate precursor (the molecule just before)? What reaction created that group?
Fill in Reagents and Conditions: Ensure you specify the correct reagents, solvents (e.g., aqueous vs. ethanolic), temperature (e.g., heat vs. cold), and catalysts for each step.
Watch for By-products: Be aware of possible competing reactions or unwanted by-products (e.g., FRS always gives multiple substitution products).

Example Logic: If you need to make a primary amine, you know two main routes:
A) Reduction of an amide (Amide $\rightarrow$ Amine via $LiAlH_4$).
B) Reduction of a nitrile (Nitrile $\rightarrow$ Amine via $LiAlH_4$).
Now, how do you make the nitrile? From a halogenoalkane via nucleophilic substitution ($KCN$ in ethanol). This chain of logic helps devise the multi-step route!