Electrophiles

An electrophile is the partner that accepts electrons from a nucleophile. Where nucleophiles are “electron-rich attackers,” electrophiles are “electron-poor targets.” Every bond-forming step pairs a nucleophile with an electrophile. If you can spot the electrophile in an organic molecule, you can predict where the nucleophile will strike.

Electrophiles come in three main flavors: full positive charges, polarized bonds, and strained/activated systems. Knowing these categories lets you scan a molecule and immediately mark “attack here.”

Type 1: Full Positive Charges (Carbocations)

A carbocation has three bonds and an empty p orbital. The carbon wants an eighth electron to complete its octet, and any nucleophile with a lone pair will do. Carbocations are the most reactive electrophiles in organic chemistry - they react with water, alcohols, halides, even relatively unreactive pi bonds.

Carbocations are named by the number of alkyl groups attached:

• Methyl cation (CH₃⁺) - essentially nonexistent; never forms in solution.
• Primary (1°) - CH₂⁺ with one R - very unstable, forms rarely.
• Secondary (2°) - CHR₂⁺ - borderline; forms in some SN1 reactions.
• Tertiary (3°) - CR₃⁺ - common intermediate in SN1/E1; stable enough to exist for microseconds.
• Resonance-stabilized cations (allyl, benzyl) - extra-stable because the positive charge delocalizes.

Section 4.9 explores carbocation stability in detail. For now, know that more substituted carbons stabilize a positive charge better, so tertiary cations appear in mechanisms while primary cations almost never do.

Type 2: Polarized Sigma Bonds

The most common electrophiles in MCAT reactions are carbons bonded to electronegative atoms. The electronegative atom pulls electron density toward itself, leaving the carbon partially positive (δ⁺). That partial positive charge is a target for nucleophiles.

Common polarized electrophiles:

Alkyl halides and sulfonate esters are the workhorse electrophiles for substitution and elimination reactions. The halogen or sulfonate pulls electrons, the carbon becomes δ⁺, and the nucleophile (or base) arrives.

Type 3: Polarized Pi Bonds (Carbonyls)

The C=O carbonyl is the single most important electrophile in organic chemistry. The oxygen is much more electronegative than carbon and holds the pi electrons closer to itself, leaving the carbon with significant δ⁺ character. Every reaction of aldehydes, ketones, carboxylic acids, esters, amides, and acid halides starts with a nucleophile attacking that electrophilic carbon.

Carbonyl electrophilicity depends on what else is attached:

• Acyl halides (R-CO-Cl, R-CO-Br): most electrophilic. Two electron-withdrawing groups pull hard on the carbonyl carbon.
• Anhydrides (R-CO-O-CO-R): very electrophilic. Still strong withdrawal from both sides.
• Aldehydes (R-CO-H): quite electrophilic. Only the alkyl and H groups, neither donating strongly.
• Ketones (R-CO-R’): less electrophilic. Two alkyl groups donate electron density to the carbonyl carbon, softening the δ⁺.
• Esters (R-CO-OR’): moderately less electrophilic. The second oxygen donates a lone pair by resonance, stabilizing the C=O and reducing the partial positive on the carbonyl carbon.
• Amides (R-CO-NR₂’): least electrophilic of common carbonyls. The nitrogen lone pair strongly donates into the C=O system, essentially neutralizing the electrophilicity of the carbonyl carbon. Amides are very unreactive.

This order - acyl halides > anhydrides > aldehydes ≈ ketones > esters > amides - is the reactivity ladder of carboxylic acid derivatives, covered in Ch 9.

Type 4: Polarized Pi Bonds in Alpha-Beta Unsaturated Systems

A C=C double bond alpha-beta to a carbonyl (an enone) has a SECOND electrophilic site - the beta-carbon. The carbonyl conjugates with the C=C pi bond, placing partial positive character on both the carbonyl carbon AND the beta-carbon. Soft nucleophiles prefer the beta-carbon (1,4-addition, also called Michael addition); hard nucleophiles prefer the carbonyl carbon (1,2-addition).

Enones are covered in Ch 7. For now: any alpha-beta unsaturated carbonyl has TWO electrophilic sites, and the choice between them depends on the nucleophile’s hardness.

Scanning a Molecule for Electrophiles

On an MCAT passage, train yourself to mark electrophilic carbons in a couple of seconds:

1. Any C bonded to a halogen (F, Cl, Br, I) is electrophilic.
2. Any C double-bonded to O (carbonyl) is electrophilic at that C.
3. Any C bonded to OR (ether) is only electrophilic after protonation of the O - usually not a direct target.
4. Any C bonded to a strong-acid-derived leaving group (OTs, OMs, OTf) is electrophilic.
5. The beta-carbon of an alpha-beta unsaturated carbonyl is electrophilic (Michael acceptor).
6. Carbocations, if present, are electrophilic everywhere (attack on the positive carbon).

Activated Electrophiles

Sometimes a neutral molecule is only a weak electrophile until something activates it:

• Acid catalysis protonates a carbonyl oxygen, pulling even more electron density off the carbon and making it a much stronger electrophile. This is why acetal formation, ester hydrolysis, and enol tautomerization are often acid-catalyzed.
• Lewis acid catalysis (BF₃, AlCl₃, FeBr₃) coordinates to a lone pair on the electrophile, pulling electron density off just like a proton would.
• Oxidation converts weak electrophiles (alcohols, amines) into stronger ones (carbonyls, imines). This is part of why oxidation is such a useful synthetic move.

Electrophile vs. Lewis Acid - Same Thing Under Two Names

In older literature and in inorganic chemistry, “Lewis acid” is the standard term. In organic chemistry, “electrophile” is the standard term. They describe the same species: electron-pair acceptors. A carbonyl carbon, a carbocation, and BF₃ are all electrophiles AND Lewis acids. Which word you use depends on your lens: mechanism (electrophile) vs. general bonding (Lewis acid).

Next: leaving groups - the molecules that have to walk away for a substitution or elimination to work.