Organic Structure & Electron Movement · Section 14 of 64

Electrophiles

Practice this — interactive lesson

If the nucleophile brings the electrons, the electrophile is what receives them. Finding the electrophile in a reaction mixture tells you where the arrow's head goes — and since most organic reactions form exactly one new bond per step, identifying the nucleophile and the electrophile means you have already worked out where that bond appears.

What it is

An electrophile — "electron-loving," though electron-poor is the more useful description — is a species that can accept an electron pair. It qualifies if it has any of three things: a full positive charge, a partial positive charge from an adjacent electronegative atom, or an empty orbital with room for a pair.

Note that a full positive charge is not required, and in most of the course it is absent. The overwhelming majority of electrophilic carbons you will attack are perfectly neutral atoms with a δ+ they acquired from a neighbor.

Worked example — a ketone

In acetone, (CH₃)₂C=O, oxygen is much more electronegative than carbon and pulls the C=O bonding electrons toward itself. The carbonyl carbon is left δ+ and the oxygen δ−.

So the carbon is the electrophilic atom — the site a nucleophile attacks — even though the oxygen is the atom that looks more interesting. The oxygen is electron-rich; it gets protonated by acids, not attacked by nucleophiles.

Both atoms have a formal charge of zero. The polarization is entirely an electronegativity effect, which is why formal charge alone is not enough to find electrophiles.

OCH₃CH₃Cδ+δ−a nucleophile attacks HEREelectron-RICH — acids protonate it,nucleophiles do not attack it
Acetone, and the trap in every carbonyl reaction. Oxygen is the atom that looks interesting — two lone pairs, a partial negative charge — and it is the wrong answer. Oxygen holds the electrons; carbon is the one that is short of them, and carbon is where a nucleophile goes. Ask which atom is electron-deficient, never which one has a symbol drawn beside it.Both atoms have a formal charge of ZERO. The polarization is pure electronegativity — which is exactly why formal charge alone will not find you an electrophile.

The main families

Polarized carbon. Any carbon bonded to something more electronegative: C–Br, C–Cl, C–OTs, and the carbonyl carbon of every aldehyde, ketone, ester, amide and acid chloride. In CH₃–Br it is the carbon that is electrophilic, not the bromine, even though bromine is the atom that eventually leaves.

Carbocations. A full positive charge combined with a genuinely empty p orbital makes these among the strongest electrophiles in the course. They are the reactive intermediates of SN1, E1, and alkene addition, and their relative stability is the theme of Module 6.

The proton, H⁺. The simplest electrophile there is — no electrons at all, one empty 1s orbital. It grabs the nearest available lone pair or pi bond, which is the whole of acid–base chemistry in Module 3 and the initiating step of a great many mechanisms.

Lewis acids. BF₃ has only six electrons on boron and a genuinely empty p orbital; AlCl₃, FeBr₃ and ZnCl₂ are similar. "Lewis acid" is simply the acid–base vocabulary for an electron-pair acceptor, which is to say, for an electrophile. The same species gets called an electrophile when we are talking about mechanism and a Lewis acid when we are talking about acidity; they are one idea with two names.

Polarized carbonCBrδ+δ−the CARBON is the target,not the bromineCarbocationRRC+full charge AND an emptyp orbital — strongest hereThe protonH+no electrons at all,one empty 1s orbitalA Lewis acidFFFBBF₃, AlCl₃, FeBr₃ — sixelectrons and a hole
Four ways to be short of electrons. Notice that the first — an ordinary neutral carbon made δ+ by the atom next door — covers the great majority of what you will attack in this course, and carries no charge at all. "Lewis acid" and "electrophile" are the same thing under two names: one is the acid–base word for it, the other the mechanism word.Only one of these four carries a full positive charge. Electron-poor is the requirement; a plus sign is just one way of being it.

Induced-dipole electrophiles. Even a perfectly nonpolar molecule can become electrophilic on approach. Br₂ has no permanent dipole, but as it nears an alkene's electron-rich pi cloud, the repulsion pushes the Br–Br electrons toward the far bromine, inducing a temporary δ+ on the near one. That transient electrophile is what the alkene attacks, and it is why bromine adds to alkenes at all (Module 7).

Br₂ on its ownBrBridentical atoms, no dipole,not an electrophile at allBr₂ approaching an alkeneCCthe π cloud, electron-richBrBrδ+δ−the alkene’s electrons repelthe Br–Br pair toward the farbromine, so the near one goes δ+
Bromine is two identical atoms sharing a pair down the middle — no dipole, nothing to attack. Then an alkene comes close, and its pi electrons push the Br–Br pair away toward the far bromine. For as long as that lasts, the near bromine is electron-poor and the alkene has something to attack. This is worth holding onto beyond Module 7: electrophilicity is a role in a pairing, not a fixed property a molecule carries around.The electrophile did not exist until the nucleophile arrived and made one.

Making an electrophile stronger

Electrophilicity can be tuned, and synthetic chemistry does this constantly. Protonation is the most common method: protonating a carbonyl oxygen gives a cation whose resonance structures place a full positive charge on the carbon, making it enormously more electrophilic than the neutral ketone. This is why so many carbonyl reactions are run under acid catalysis.

Coordination to a Lewis acid does the same job without a proton, and works where acid would destroy the substrate. Converting to a better leaving group is the third route — turning an alcohol into a tosylate does not change the carbon's charge much but makes the whole system far more reactive toward nucleophiles, for reasons the next section takes up.

The electrophilic atom is usually not the one that looks charged. In a protonated carbonyl the plus sign is drawn on oxygen, but the attack happens at carbon. In C–Br the bromine gets all the attention, but the carbon is the target. When hunting for an electrophile, ask which atom is electron-deficient, not which one has a symbol next to it.
Once you have located the nucleophile and the electrophile, you know where the new bond forms: the nucleophile's electrons go to the electrophile's electron-poor atom. Almost every mechanism in this course is that sentence, repeated with different molecules — which is why reading a molecule for electron density is worth more than memorizing reactions.

Electrophilicity is relative

Nothing is an electrophile or a nucleophile in the abstract; both labels describe a role in a particular reaction. Water has lone pairs and acts as a nucleophile toward a carbocation, but its hydrogens are electrophilic enough to be attacked by a strong base. An alkene is a nucleophile toward Br₂ but becomes electrophilic at the beta carbon once it is conjugated to a carbonyl.

The question is never "is this molecule an electrophile" but "in this pairing, which side is electron-poor." That comparison is what the next section, and the ranking of electron-rich against electron-poor, is for.

What carries forward

The electrophilic carbon of a C–LG bond drives all of Module 6. The electrophilic carbonyl carbon drives Modules 9, 10 and 11. Electrophilic aromatic substitution in Module 13 is named for this idea outright. And the generation of unusually strong electrophiles — acylium ions, bromonium ions, protonated carbonyls — is a recurring theme every time a reaction needs a push.