Acids & Bases Section 19 of 123

Conjugate acids/bases

the why on every wrong step

Practice this lesson

10 min readfree

When an acid gives away its proton, what is left behind can take a proton back, so it is a base. Every acid therefore has a partner base, and every base has a partner acid. This page is about those partners, and about how the strength of one sets the strength of the other.

What "conjugate" means

Take hydrogen chloride, HCl. When it gives its proton (H⁺) to a base, what remains is chloride ion, Cl⁻. HCl and Cl⁻ contain the same atoms apart from that one proton. Two species related in this way form a conjugate acid–base pair.

Each member of the pair has its own name, first used in Brønsted acids/bases. The member with the extra proton is the conjugate acid, and the member without it is the conjugate base. So Cl⁻ is the conjugate base of HCl, and HCl is the conjugate acid of Cl⁻.

Other pairs work the same way. H₃O⁺ and H₂O differ by one proton, and so do H₂O and HO⁻, NH₄⁺ and NH₃, and acetic acid (CH₃COOH) and acetate ion (CH₃COO⁻). In every pair the two species differ by one hydrogen and one unit of charge, and in nothing else.

acidconjugate baseClH−H⁺Cl−OHHH+−H⁺OHHNHHHH+−H⁺NHHHOOH−H⁺OO−
In each row the highlighted hydrogen leaves as H⁺. The electron pair that held it stays behind as a lone pair, so every charge on the right is one lower than the charge beside it on the left.

Water turns up in that list twice. It can gain a proton to become hydronium ion, H₃O⁺, or lose one to become hydroxide ion, HO⁻. So water is the conjugate base in one pair and the conjugate acid in the other.

Brønsted acids/bases called a species like this, one that can act as either an acid or a base, amphoteric. Which role it plays depends on its partner. Water takes a proton from HCl, so there it is a base. It gives a proton to ammonia, so there it is an acid. Alcohols and amines are amphoteric as well, as the worked example further down shows, and so are the amino acids of Biomolecules.

OHHH+hydronium, H₃O⁺conjugate acid of water+H⁺OHHwater, H₂Oa base going up,an acid going down−H⁺OH−hydroxide, HO⁻conjugate base of water
Hydronium and hydroxide sit two protons apart, with water between them, so they are not a conjugate pair.
Worked example — labeling all four species

CH₃COOH + NH₃ ⇌ CH₃COO⁻ + NH₄⁺

Acetic acid loses a proton, so it is the acid, and acetate is its conjugate base. Ammonia gains that proton, so it is the base, and ammonium ion is its conjugate acid.

Every proton transfer fills all four roles, so it always contains two conjugate pairs, with one member of each pair on each side of the arrow. To find a species' partner, look on the other side for the one that differs from it by exactly one proton.

pair 1pair 2OOH+NHHHacidbaseOO−+NHHHH+conjugate baseconjugate acid
Read each pair down its column: acetic acid above acetate, ammonia above ammonium. The highlighted hydrogen is the one that moves. Acetate and ammonia also sit on opposite sides of the arrow, but they are different molecules, not one molecule with and without a proton.

The inverse relationship

Start with HCl in water. HCl gives its proton to water almost completely: once the solution settles, nearly every HCl molecule has become Cl⁻, and hardly any HCl is left. An acid that hands over its proton this completely is a strong acid.

Now read the same reaction backwards. For HCl to form again, Cl⁻ would have to take a proton back from H₃O⁺, and it almost never does. A base that takes a proton this reluctantly is a weak base. So the strong acid HCl has a very weak conjugate base.

Acetic acid behaves differently. In water it gives up only a small fraction of its protons, so it is a weak acid. Its conjugate base, acetate, takes a proton back readily, which is why most of the acetic acid stays as CH₃COOH. So acetate is a much stronger base than chloride, though still only a moderate one.

Both cases follow one rule: the stronger an acid, the weaker its conjugate base, and the weaker an acid, the stronger its conjugate base. This is not a separate fact to memorize. An acid is strong because it lets go of its proton easily, and a base left behind by such an acid has little pull on that proton. Little pull on a proton is exactly what "weak base" means.

The rule holds for every acid, from the strongest to the weakest. HCl is a very strong acid, and chloride is so weak a base that a solution of table salt, NaCl, is not basic at all. Water is a very weak acid, and its conjugate base, hydroxide, is a strong and reactive base. Ammonia is a weaker acid still: it gives up an N–H proton even less readily than water gives up an O–H proton. Its conjugate base, amide ion (⁻NH₂), is one of the strongest bases in this course.

acidconjugate baseHClvery strong acidCl⁻very weak baseCH₃COOHweak acidCH₃COO⁻moderate baseH₂Overy weak acidHO⁻strong baseNH₃extremely weak acid⁻NH₂very strong base↑ stronger acidstronger base ↓
Each row is one conjugate pair. Read the left arrow for the acids and the right arrow for the bases.
Preview. Nothing on this page depends on this box. In How Reactions Happen, a group that breaks away from a molecule and takes the bonding electrons with it is called a leaving group. Good leaving groups are stable on their own, which makes them weak bases and so, by the rule above, the conjugate bases of strong acids. Chloride is one. So "conjugate base of a strong acid", "weak base", "stable anion" and "good leaving group" turn out to describe one property, and Substitution & Elimination moves between those names constantly.

One ranking, read in both directions

Sodium amide, NaNH₂, is a reagent chemists use to pull protons off very weak acids. The part that does the work is amide ion, the very strong base met above as the conjugate base of ammonia.

All the very strong bases in this course follow the same pattern. Each is the conjugate base of an extremely weak acid, and the figure draws each one beside its acid.

  • LDA (lithium diisopropylamide) supplies the conjugate base of diisopropylamine, an amine with two isopropyl groups on its nitrogen.
  • NaNH₂ (sodium amide) supplies amide ion, the conjugate base of ammonia.
  • NaH (sodium hydride) supplies hydride ion, H⁻, the conjugate base of hydrogen gas, H₂.
  • Butyllithium (BuLi) reacts as a source of the butyl anion, the conjugate base of butane.
extremely weak acidvery strong baseNH−H⁺N−Li⁺diisopropylaminein LDANHHH−H⁺NHH−Na⁺ammoniain NaNH₂HH−H⁺H−Na⁺hydrogen, H₂in NaHH−H⁺−Li⁺butanein butyllithium
The highlighted hydrogen is the proton each acid would lose. The metal ion only balances the charge. Butyllithium really has a polar C–Li bond rather than a free anion, but it reacts as if it carried the butyl anion.

This is what the pairing buys you. A list of acids ranked from strongest to weakest is also a list of their conjugate bases, ranked from weakest to strongest. You learn the ranking once and read it in either direction. When you need a very strong base, look for the conjugate base of a very weak acid.

Worked example — the conjugates of an alcohol and an amine

Organic molecules follow the same definition, and many of them have both a conjugate acid and a conjugate base. Ethanol and ethylamine show both directions.

Ethanol, CH₃CH₂OH. Remove the O–H proton and you get its conjugate base, ethoxide ion, CH₃CH₂O⁻. Add a proton to one of the oxygen's lone pairs instead and you get its conjugate acid, CH₃CH₂OH₂⁺. That is an oxonium ion, an ion whose positively charged oxygen has three bonds.

Ethylamine, CH₃CH₂NH₂. Add a proton to the nitrogen's lone pair and you get its conjugate acid, the ammonium ion CH₃CH₂NH₃⁺. Remove an N–H proton and you get its conjugate base, CH₃CH₂NH⁻, an amide anion. (Despite the name, this is not the amide functional group, in which nitrogen is bonded to a C=O.)

conjugate acidthe neutral moleculeconjugate baseOHH++H⁺OH−H⁺O−an oxonium ionethanolethoxide ionNHHH++H⁺NHH−H⁺NH−an ammonium ionethylaminean amide anion
In each conjugate acid, the highlighted hydrogen has bonded to one of the neutral molecule’s lone pairs. Compare the lone pairs and the charge in each drawing with the neutral molecule in the middle.

Of the two ethylamine conjugates, the ammonium ion is the one you will meet often, because an amine picks up a proton from almost any acid. The amide anion is rare. Ethylamine is an extremely weak acid, so removing its N–H proton takes a base as strong as butyllithium.

Preview. The same operation applies to groups that later chapters protonate or deprotonate. Adding a proton to the oxygen of a ketone such as acetone gives a protonated carbonyl. Removing a proton from a carbon next to the C=O gives an anion that Enolate Chemistry calls an enolate. Removing the hydrogen from the end carbon of a terminal alkyne (an alkyne whose C≡C is at the end of the chain, with an H on the last carbon) gives an acetylide. Why those particular hydrogens come off is a question for Factors affecting acidity. Whatever the group, add or remove one H and one unit of charge, and change nothing else.

conjugate acidthe neutral moleculeconjugate baseOH++H⁺O−H⁺O−protonated carbonylacetone, a ketoneenolate (preview)H−H⁺−propyne, a terminal alkyneacetylide (preview)
The same one-proton operation on a ketone and on a terminal alkyne. The negative charge in the enolate is also shared with the oxygen by resonance; Enolate Chemistry draws both forms.
A conjugate base is not automatically negative. Losing a proton lowers the charge by one; it does not set the charge to −1. H₃O⁺ is an acid, and its conjugate base is neutral water. NH₄⁺ is an acid, and its conjugate base is neutral ammonia. Going the other way, the conjugate acid of a neutral base is a cation: water gives H₃O⁺, and an amine gives an ammonium ion. A common slip is to give every conjugate base a minus sign and every conjugate acid no charge at all. Instead, write down the charge you started with, then add or subtract one.
"Conjugate" describes a relationship, not a property. Chloride is not "a conjugate base" in the abstract; it is the conjugate base of HCl. Every base is the conjugate base of something. Asking "is this a conjugate base?" is like asking "is this a sibling?": the answer depends on naming the other member of the pair.
Conjugate pairs differ by one proton, never by two. Sulfuric acid, H₂SO₄, has two acidic protons and gives them up one at a time. HSO₄⁻ is the conjugate base of H₂SO₄, and SO₄²⁻ is the conjugate base of HSO₄⁻. But SO₄²⁻ is not the conjugate base of H₂SO₄, because the two differ by two protons. Each step is its own conjugate pair, and the pKa section gives each step its own number.

Preview: the numbers behind the ranking

Brønsted acids/bases introduced the number chemists use to rank acids, the pKa: the lower it is, the stronger the acid. The next section, pKa, explains where it comes from. The table lists the values for the acids on this page, strongest acid first. It is here for reference only, and nothing on this page depends on it.

AcidConjugate baseApproximate pKa
HClCl⁻−7
CH₃CH₂OH₂⁺CH₃CH₂OH−2
CH₃COOHCH₃COO⁻4.76
NH₄⁺NH₃9.2
CH₃CH₂NH₃⁺CH₃CH₂NH₂10.7
H₂OHO⁻15.7
CH₃CH₂OHCH₃CH₂O⁻16
diisopropylaminediisopropylamide (in LDA)36
H₂H⁻36
NH₃⁻NH₂38
CH₃CH₂NH₂CH₃CH₂NH⁻38
butanebutyl anion50

The numbers also settle the worked example with acetic acid and ammonia. Acetic acid (4.76) gives up its proton far more readily than ammonium ion (9.2), so that equilibrium lies almost completely on the right, by about 3 × 10⁴ to one, as Brønsted acids/bases worked out. The pKa section, next, shows how to make predictions like this in general.

The table also puts a number on the two conjugates of ethanol. Its conjugate acid (−2) and ethanol itself (16) differ by about 18 pKa units, so adding one proton makes the molecule about 10¹⁸ times more acidic.

What carries forward

The conjugate relationship lets one ranking serve for acid strength and base strength, and later for leaving-group ability too. The next section, pKa, puts numbers on that ranking. The one after it, Factors affecting acidity, explains which features of a structure decide where a species lands on it.