Welcome to Organic Chemistry II!
Welcome to one of the most exciting parts of your A Level Chemistry journey! In Organic Chemistry I, you learned the basics of chains and simple functional groups. Now, we are diving deeper. We’ll explore why some molecules have a "left" and "right" hand (chirality) and look at the versatile world of carbonyl compounds and carboxylic acids.
Organic chemistry is like building with LEGO bricks; once you understand how the pieces (functional groups) click together, you can predict how almost any molecule will behave. Don’t worry if some mechanisms look like "spaghetti" at first—we will break them down step-by-step!
Topic 17A: Chirality – The "Handedness" of Molecules
Have you ever noticed that your left hand and right hand are mirror images, but you can’t perfectly overlap them? This is exactly what chirality is in chemistry.
1. Chiral Centres and Enantiomers
A molecule is chiral if it has a chiral centre. This is a carbon atom attached to four different groups. We often mark this carbon with an asterisk (*).
- Enantiomers: These are two molecules that are non-superimposable mirror images of each other.
- Asymmetric Carbon Atom: Another name for the chiral centre carbon.
Analogy: Think of your hands. They are mirror images, but no matter how you turn them, a right-hand glove won't fit a left hand. They are non-superimposable.
2. Optical Activity
Chiral molecules are "optically active." This means a single enantiomer can rotate the plane of plane-polarised monochromatic light.
- One enantiomer rotates the light clockwise (+).
- The other enantiomer rotates it anticlockwise (-) by the exact same amount.
3. Racemic Mixtures
A racemic mixture (or racemate) contains equal amounts (a 50/50 mix) of both enantiomers. Because they rotate light in opposite directions by the same amount, the effects cancel out.
Key Point: A racemic mixture is not optically active and will not rotate plane-polarised light.
Quick Review: Why do we care about optical activity in mechanisms?
We use it as evidence for how reactions happen!
1. \( S_N2 \) Mechanisms: Usually result in a single enantiomer (the molecule "flips" like an umbrella in the wind), so the product is optically active.
2. \( S_N1 \) Mechanisms: Start by forming a flat (planar) carbocation. The nucleophile can attack from either side equally, creating a racemic mixture. The product will not be optically active.
Key Takeaway: Chirality requires a carbon with 4 different groups. Single enantiomers rotate light; racemic mixtures don't.
Topic 17B: Carbonyl Compounds (Aldehydes and Ketones)
Carbonyl compounds contain the \( \text{C=O} \) group. They are found in everything from nail polish remover to the smell of cinnamon!
1. Identifying the Groups
- Aldehydes: The \( \text{C=O} \) is at the end of the chain (attached to at least one Hydrogen). Generic formula: \( \text{RCHO} \).
- Ketones: The \( \text{C=O} \) is in the middle of the chain (attached to two Carbon groups). Generic formula: \( \text{RCOR'} \).
2. Physical Properties
Boiling Points: Carbonyls have higher boiling points than alkanes (due to permanent dipole-dipole forces) but lower boiling points than alcohols. This is because carbonyl molecules cannot form hydrogen bonds with each other.
Solubility: Small aldehydes and ketones are soluble in water because they can form hydrogen bonds with water molecules.
3. Chemical Tests (Crucial for Exams!)
How do we tell them apart? Aldehydes are easily oxidised, but ketones are not.
- Tollens’ Reagent: Add to the sample and warm. Aldehydes produce a "silver mirror" on the inside of the test tube. Ketones? No change.
- Fehling’s or Benedict’s Solution: Blue solution turns into a brick-red precipitate with aldehydes. Ketones stay blue.
- Acidified Dichromate (VI) \( \text{Cr}_2\text{O}_7^{2-}/\text{H}^+ \): Turns from orange to green with aldehydes.
4. Reactions of Carbonyls
Reduction: Both can be turned back into alcohols using \( \text{LiAlH}_4 \) (lithium tetrahydridoaluminate) in dry ether.
Aldehyde \( \rightarrow \) Primary Alcohol
Ketone \( \rightarrow \) Secondary Alcohol
Nucleophilic Addition with \( \text{HCN} \):
This is a famous mechanism! Using \( \text{HCN} \) in the presence of \( \text{KCN} \), the \( \text{CN}^- \) ion attacks the \( \delta+ \) Carbon.
Did you know? This reaction increases the carbon chain length by one!
2,4-DNPH (Brady’s Reagent): This tests for the presence of a carbonyl group (both aldehydes and ketones). It forms a bright orange/yellow precipitate. We can then filter this, purify it, and measure its melting point to identify the specific compound.
Iodoform Test (Iodine in alkali): Tests for a methyl carbonyl group (\( \text{CH}_3\text{C=O} \)). A pale yellow precipitate of \( \text{CHI}_3 \) forms with a distinct "antiseptic" smell.
Key Takeaway: Use Tollens' to find aldehydes. Use 2,4-DNPH to find any carbonyl. Use Iodoform to find methyl ketones.
Topic 17C: Carboxylic Acids and Their Relatives
Carboxylic acids (\( \text{-COOH} \)) are weak acids, but they have some very strong personalities!
1. Physical Properties
Carboxylic acids have very high boiling points. Why? They can form hydrogen bonds with each other to create "dimers" (two molecules stuck together), effectively doubling their molecular mass!
2. Preparation
- Oxidation: Primary alcohols or aldehydes can be oxidised using acidified \( \text{K}_2\text{Cr}_2\text{O}_7 \) under reflux.
- Hydrolysis of Nitriles: Boil a nitrile with dilute acid (like \( \text{HCl} \)). The \( \text{-CN} \) group turns into a \( \text{-COOH} \) group.
3. Reactions of Carboxylic Acids
- Reduction: Use \( \text{LiAlH}_4 \) to go straight back to a primary alcohol. (Note: \( \text{NaBH}_4 \) isn't strong enough for this!).
- Neutralisation: They react with bases (like \( \text{NaOH} \)) to form salts (e.g., sodium ethanoate) and water.
- Phosphorus (V) Chloride (\( \text{PCl}_5 \)): This replaces the \( \text{-OH} \) with a \( \text{-Cl} \), creating an acyl chloride. Steamy white fumes of \( \text{HCl} \) are observed.
- Esterification: React a carboxylic acid with an alcohol (plus an acid catalyst) to make an ester. Esters smell fruity!
4. Acyl Chlorides – The "Reactive Cousins"
Acyl chlorides (\( \text{-COCl} \)) are much more reactive than carboxylic acids. They react vigorously at room temperature:
- + Water \( \rightarrow \) Carboxylic Acid + \( \text{HCl} \).
- + Alcohols \( \rightarrow \) Ester + \( \text{HCl} \).
- + Concentrated Ammonia \( \rightarrow \) Amide + \( \text{HCl} \).
- + Amines \( \rightarrow \) N-substituted Amide + \( \text{HCl} \).
5. Esters and Polyesters
Ester Hydrolysis:
1. Acid Hydrolysis: Reversible reaction. Gives back the acid and alcohol.
2. Alkaline Hydrolysis: Non-reversible. Gives the salt of the acid and the alcohol. This is often called saponification (how soap is made!).
Polyesters: Formed by condensation polymerisation. You need either:
- A monomer with a \( \text{-COOH} \) at both ends and another with \( \text{-OH} \) at both ends.
- A single monomer with \( \text{-COOH} \) at one end and \( \text{-OH} \) at the other.
Every time a link is made, a small molecule (usually water) is kicked out!
Key Takeaway: Carboxylic acids are great for making esters and acyl chlorides. Acyl chlorides are highly reactive "building blocks." Polyesters are made by losing water.
Don't Forget! Common Mistakes to Avoid:
- The \( \text{LiAlH}_4 \) trap: Remember it must be used in dry ether because it reacts violently with water.
- Distillation vs Reflux: Use distillation to stop at an aldehyde; use reflux to go all the way to a carboxylic acid.
- Chiral Centres: Double-check all 4 groups. A \( \text{CH}_2 \) group can never be a chiral centre because it has two identical Hydrogens.
Keep practicing those mechanisms! Don't worry if it feels tricky at first—organic chemistry is all about spotting the patterns. You've got this!