Chemistry | Organic Chemistry: Carbonyls, Carboxylic Acids and Chirality

Welcome to Carbonyl Chemistry and Chirality!

Hello future Chemist! This chapter might look daunting, but it’s actually one of the most exciting areas of organic chemistry. We are going to explore the chemistry of the highly reactive Carbonyl group (\(C=O\)) found in common substances like vinegar, nail polish remover, and even flavourings! We’ll also dive into chirality, which explains why some molecules are non-superimposable mirror images of each other – a concept essential for understanding biology and pharmaceuticals.

Don't worry if some concepts seem tricky at first. We will break them down using clear steps and relatable analogies. Let's get started!

Section 1: The Chemistry of Aldehydes and Ketones

The Carbonyl Group (\(C=O\)): Structure and Polarity

The key feature of aldehydes and ketones is the carbonyl group, \(C=O\). This bond is highly polar because Oxygen is much more electronegative than Carbon.

• The Oxygen atom pulls electrons towards itself, gaining a partial negative charge (\(\delta-\)).
• The Carbon atom becomes electron deficient, gaining a partial positive charge (\(\delta+\)).

Why is this polarity important?
The electron-deficient carbonyl carbon (\(C^{\delta+}\)) is a prime target for attack by nucleophiles (electron-rich species). This explains the characteristic reaction type of carbonyls: Nucleophilic Addition.

Key Difference: Aldehydes vs. Ketones

Both contain the \(C=O\) group, but where the group sits determines the class:

• Aldehyde: The \(C=O\) group is at the end of the carbon chain (bonded to at least one H atom). Name ends in -al (e.g., Ethanal).
• Ketone: The \(C=O\) group is in the middle of the chain (bonded to two other Carbon atoms). Name ends in -one (e.g., Propanone).

Reactions of Aldehydes and Ketones

1. Reduction (Forming Alcohols)

Aldehydes and ketones can be reduced back into alcohols using reducing agents like Sodium Tetrahydridoborate(III) (\(NaBH_4\)), typically dissolved in aqueous ethanol.

Step-by-step result:

• Aldehyde + \([H]\) \(\rightarrow\) Primary Alcohol
• Ketone + \([H]\) \(\rightarrow\) Secondary Alcohol

Analogy: Reduction is like putting the hydrogen atoms back onto the carbonyl carbon and oxygen, returning it to the alcohol state.

2. Oxidation (The Distinguishing Test)

This is vital for distinguishing between aldehydes and ketones:

• Aldehydes are easily oxidised (even by mild oxidising agents) to form Carboxylic Acids.
• Ketones are resistant to oxidation because breaking them requires breaking a C-C bond, which takes much more energy.

Two crucial tests:

a) Tollen’s Reagent (Aqueous Silver Nitrate in Ammonia)

The active oxidising agent is the diamminesilver(I) ion, \([Ag(NH_3)_2]^+\).

• Positive Test (Aldehyde): A Silver Mirror forms on the inner surface of the test tube as silver ions are reduced to metallic silver:
  \(\text{Aldehyde} + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow \text{Carboxylate Ion} + 2Ag_{(s)} + 4NH_3 + 2H_2O\)
• Negative Test (Ketone): Solution remains clear.

b) Fehling’s Solution (or Benedict’s Solution)

Contains Copper(II) ions (\(Cu^{2+}\)).

• Positive Test (Aldehyde): The clear blue solution forms a Brick Red precipitate of Copper(I) oxide (\(Cu_2O\)) as \(Cu^{2+}\) is reduced to \(Cu^+\).
• Negative Test (Ketone): Solution remains blue.

3. Nucleophilic Addition with HCN (Formation of Hydroxynitriles)

The reaction of carbonyls with Hydrogen Cyanide (HCN) forms a class of compounds called hydroxynitriles (or cyanohydrins). This reaction increases the carbon chain length by one, which is useful in synthesis.

Because HCN is extremely toxic, the reaction is usually carried out using potassium cyanide (KCN) followed by dilute acid, or by generating HCN in situ (in the reaction mixture).

The Mechanism: Nucleophilic Addition (The essential mechanism for this chapter!)

The key nucleophile is the cyanide ion, \(CN^-\).

Step 1: Nucleophilic Attack

The electron pair on the \(CN^-\) ion attacks the electron-deficient carbonyl carbon (\(C^{\delta+}\)). The \(\pi\)-electrons in the \(C=O\) bond instantaneously move entirely onto the highly electronegative oxygen atom, forming an unstable intermediate alkoxide ion.

$$ R_2C=O + CN^- \rightarrow R_2C(O^-)CN $$

Step 2: Protonation

The lone pair on the intermediate oxygen ion attracts a proton (\(H^+\)) from the acid (or water), completing the hydroxynitrile product.

$$ R_2C(O^-)CN + H^+ \rightarrow R_2C(OH)CN $$

Section 2: Carboxylic Acids and Their Derivatives

Carboxylic Acids: Structure and Acidity

Carboxylic acids contain the carboxyl group, \(-COOH\). Examples include Methanoic acid (Formic acid) and Ethanoic acid (Acetic acid, found in vinegar).

Carboxylic acids are weak acids—they partially dissociate in water:

$$ RCOOH_{(aq)} \rightleftharpoons RCOO^-_{(aq)} + H^+_{(aq)} $$

Why are they acidic?
While they are weak acids, they are much stronger acids than alcohols. This is because the resulting ion, the carboxylate ion (\(RCOO^-\)), is very stable.

The secret is resonance stabilisation: The negative charge on the oxygen atom is delocalised (spread out) over both oxygen atoms in the carboxylate ion. Spreading the charge makes the ion more stable, meaning the equilibrium position is further to the right, releasing more \(H^+\).

Reactions of Carboxylic Acids

As acids, they react with bases, carbonates, and metals to form salts:

1. Reaction with Carbonates: Forms a salt, water, and carbon dioxide (effervescence - the classic test for carboxylic acids).
   \(2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COO^-Na^+ + H_2O + CO_2\)
2. Reaction with Bases: Neutralisation reaction forming a salt and water.

Esterification (Forming Esters)

Carboxylic acids react with alcohols in the presence of a strong acid catalyst (usually concentrated sulfuric acid, \(H_2SO_4\)) to form an ester and water.

$$ RCOOH + R'OH \rightleftharpoons RCOOR' + H_2O $$

Note: This is an equilibrium reaction. To maximise the yield of the ester, water must be removed, or an excess of one reactant (often the cheaper alcohol) must be used.

Acyl Chlorides (Acid Chlorides) and Acid Anhydrides

These compounds are known as carboxylic acid derivatives. They are highly reactive due to the excellent leaving groups attached to the carbonyl carbon (Cl or \(RCOO^-\)).

The general formula for an Acyl Chloride is \(RCOCl\). They are named by replacing -oic acid with -oyl chloride (e.g., Ethanoyl chloride, \(CH_3COCl\)).

High Reactivity of Acyl Chlorides

The \(C=O\) carbon in acyl chlorides is even more electron-deficient (\(\delta+\)) than in carboxylic acids because chlorine is highly electron-withdrawing. Therefore, acyl chlorides react very readily with nucleophiles.

Acyl chlorides are the most reactive carboxylic acid derivative.

Nucleophilic Acyl Substitution Reactions

Acyl chlorides react violently or readily with four main types of nucleophiles, replacing the Cl atom (which is a great leaving group) with the nucleophile:

1. Reaction with Water (Hydrolysis): Very vigorous, producing the carboxylic acid and misty fumes of HCl.
   \(RCOCl + H_2O \rightarrow RCOOH + HCl\)
2. Reaction with Alcohol: Forms an ester (a much faster and non-reversible way to make esters compared to using the carboxylic acid).
   \(RCOCl + R'OH \rightarrow RCOOR' + HCl\)
3. Reaction with Ammonia: Forms a primary amide and ammonium chloride.
   \(RCOCl + 2NH_3 \rightarrow RCONH_2 + NH_4Cl\)
4. Reaction with Primary Amines: Forms a secondary amide.
   \(RCOCl + 2R'NH_2 \rightarrow RCONHR' + R'NH_3^+Cl^-\)

Section 3: Chirality and Optical Isomerism

This is where we leave functional groups temporarily and look at the 3D structure of molecules. Chirality is absolutely central to biology—most natural products (like amino acids and sugars) are chiral.

Chiral Centres (The Origin of Chirality)

A molecule is chiral if it is non-superimposable upon its mirror image. The cause of chirality in organic molecules is often the presence of a chiral centre (or asymmetric carbon atom).

Definition of a Chiral Centre:
A carbon atom bonded to four different groups.

Analogy: Your hands are chiral. They are mirror images of each other, but you cannot superimpose them—if you put your right hand on top of your left hand, they won't match up (the thumb points in the wrong direction).

Optical Isomerism (Enantiomers)

Isomers that are non-superimposable mirror images of each other are called enantiomers (a type of stereoisomerism).

Enantiomers have identical physical and chemical properties (like boiling point and reactivity with non-chiral substances) except for one crucial difference: their interaction with plane-polarised light.

Optical Activity:
Enantiomers are optically active, meaning they rotate the plane of vibration of plane-polarised light.

• One enantiomer rotates the plane clockwise (dextrorotatory, designated \(+\) or \(d\)).
• The other enantiomer rotates the plane anti-clockwise (laevorotatory, designated \(-\) or \(l\)).
• They rotate the light by exactly the same magnitude (amount), but in opposite directions.

Racemates (Racemic Mixtures)

A racemate or racemic mixture is an equimolar (50:50) mixture of the two enantiomers.

Crucial Fact: A racemate is optically inactive because the rotation caused by one enantiomer is exactly cancelled out by the opposite rotation caused by the other.

How are Racemates formed? (Linking back to Carbonyls!)

When a nucleophile attacks a planar carbonyl group (\(C=O\)), the attack is equally likely to happen from the "top" side or the "bottom" side (50% probability for each).

If the addition reaction creates a new chiral centre, attacking from the top forms one enantiomer, and attacking from the bottom forms the mirror-image enantiomer. The result is always a racemate (50:50 mixture), which is optically inactive.

Example: Addition of \(CN^-\) to Ethanal creates a chiral centre and thus forms a racemic mixture of 2-hydroxypropanenitrile.

Congratulations! You have successfully navigated the complex world of carbonyl chemistry, reactivity, and 3D isomerism. Remember to focus on the mechanisms—understanding the polarity of the \(C=O\) bond is the key to unlocking almost all of these reactions. Keep practicing those structures!