Brønsted-Lowry Proton-Transfer Reactions Chemistry Tutorial

Key Concepts

• A proton⁽¹⁾ (H⁺) is a hydrogen atom (H) that has lost its electron (e^-):

  H → H⁺ + e^-

  Proton (or hydron) is another name for the hydrogen ion or hydrogen(1+)

• A Brønsted-Lowry acid is a proton donor:
  

  acid	produces	a proton	and	its conjugate base
  HA	→	H+	+	A-

• A Brønsted-Lowry base is a proton acceptor:
  

  base	accepts	a proton	producing	its conjugate acid
  B-	+	H+	→	HB

• Proton-transfer reaction: a proton is transferred from a Brønsted-Lowry acid to a Brønsted-Lowry base.
  

  Brønsted-Lowry acid	+	Brønsted-Lowry base	produces	conjugate base	+	conjugate acid
  HA	+	B-	→	A-	+	HB
  A proton, H+, transferred from the acid, HA, to the base, B-
  The products of the proton-transfer reaction are: 1. The conjugate base of the Brønsted-Lowry acid 2. The conjugate acid of the Brønsted-Lowry base.

• Proton-transfer reactions include, but are not limited to, reactions of Arrhenius acids and bases such as:

  ⚛ Hydrolysis reactions (reactions with water) can be treated as proton-transfer reactions

  ⚛ Neutralisation reactions (reactions between Arrhenius acids and bases) can be treated as proton-transfer reactions

• Proton-transfer reactions also include reactions of non-Arrhenius acids and bases and in which water is not the solvent.
• Not every reaction that involves hydrogen atoms will be a proton-transfer reaction.

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Brønsted-Lowry Proton-Transfer Reactions in Aqueous Solutions : Hydrolysis Reactions

Water is a common solvent, particularly for inorganic⁽²⁾ chemical reactions.

Hydrolysis⁽³⁾ is the name given to the reaction that occurs between water and another species.

Water (H₂O) is an amphiprotic substance, it can act as either a:

• Brønsted-Lowry acid by donating a proton (H⁺) to produce its conjugate base (OH^-)
• Brønsted-Lowry base by accepting a proton (H⁺) to produce its conjugate acid (H₃O⁺)

When a Brønsted-Lowry acid dissolves in water, water (H₂O) acts as a Brønsted-Lowry base, so a proton (H⁺) is transferred from the acid to a water molecule.

When a Brønsted-Lowry base dissolves in water, water (H₂O) acts as a Brønsted-Lowry acid, so a proton is transferred from a water molecule to the base.

In the discussions below, we will consider that the proton-transfer reaction occurs in stages so that we can model what is happening during the proton-transfer reaction.

Dissolving a Brønsted-Lowry Acid in Water (hydrolysis of an acid)

1. When an acid, HA, dissolves in water, H₂O, it dissociates⁽⁴⁾ to form an anion, A^-, and a proton, H⁺:

   H A

   →

   A-

   +

   H+

   
   Notice that the electron contributed by the hydrogen atom to the bonding pair of electrons in the acid HA remains behind to form the anion A^- and that the hydrogen "atom" no longer has an electron, it is the hydrogen ion H⁺ (a proton).
2. The proton (H⁺) bonds to the oxygen atom of a water molecule (H₂O) to form the oxidanium⁽⁵⁾ (oxonium or hydronium or hydroxonium) ion, H₃O⁺:

   H+

   +

   H O H

   →

   [   H   ]+ H O H

   Notice that the central oxygen atom contributes both electrons to the bonding pair of electrons between the oxygen atom and the proton. This type of covalent bond is known as a coordinate covalent bond (or a dative bond).

3. We can then write an overall chemical equation to represent the transfer of the proton from the Brønsted-Lowry acid (HA) to a water molecule (H₂O) which is acting as a Brønsted-Lowry base:

   acid dissociates producing a proton :	HA(aq)			→	H+(aq)	+	A-(aq)
   base (water) accepts the proton :	H+(aq)	+	H2O(l)	→	H3O+(aq)
   overall equation for the transfer of a proton from an acid to water:	HA(aq)	+	H2O(l)	→	H3O+(aq)	+	A-(aq)

   By definition, water is acting as a Brønsted-Lowry base because it has accepted a proton (H⁺) to form the oxidanium (oxonium or hydronium or hydroxonium) ion, H₃O⁺.
   H₃O⁺ is the conjugate acid of water, H₂O, when it is acting as a Brønsted-Lowry base.

   Brønsted-Lowry acid	+	Brønsted-Lowry base	→	conjugate base (of acid HA)	+	conjugate acid (of base H2O)
   H . . A	+	H .. :O :H ..	→	. . A-	+	[ H .. ]+ H :O :H ..
   acid	+	water	→	conjugate base	+	oxidanium (oxonium or hydronium or hydroxonium) ion

Examples of proton transfer from an acid to water when a Brønsted-Lowry acid dissolves in water:

Note: we are indicating that the proton-transfer reaction occurs in the solvent water by including the states of matter in round brackets:

⚛ (l) indicates that water is present as a liquid

⚛ (aq) indicates that the species is present in aqueous solution (that is, water is the solvent)

Dissolving a Brønsted-Lowry Base in Water (hydrolysis of a base)

1. Water molecules, H₂O, self-dissociate, this means that in pure liquid water the water molecules exist in equilibrium with protons, H⁺, and hydroxide ions, OH^- as shown in the chemical equation below:

   H₂O_(l) ⇋ H⁺_(aq) + OH^-_(aq)

   Water can therefore act as a Brønsted-Lowry acid by donating a proton to a Brønsted-Lowry base.

2. When a Brønsted-Lowry base dissolves in water, it accepts a proton from water molecules:

   B^-_(aq) + H⁺_(aq) → HB_(aq)

3. Overall reaction for the transfer of a proton from water (a Brønsted-Lowry acid) to a Brønsted-Lowry base:

   Dissociation of water produces proton:			H2O(l)	→	H+(aq)	+	OH-(aq)
   Base accepts proton	B-(aq)	+	H+(aq)	→	HB(aq)
   Proton-transfer reaction:	B-(aq)	+	H2O(l)	→	HB(aq)	+	OH-(aq)

   Brønsted-Lowry base	+	Brønsted-Lowry acid (water)	→	conjugate acid of base	+	conjugate base of water (hydroxide ion)
   :B-	+	H ..     :O :H   ..	→	H:B	+	..     :O :H-   ..

   Water, H₂O, has donated a proton, H⁺, to the Brønsted-Lowry base B^-

   By definition, water is acting as a Brønsted-Lowry acid.

   OH^- is the conjugate base of water, H₂O.

   Examples of proton transfer from water to a base when a Brønsted-Lowry base dissolves in water:

   Brønsted-Lowry base	+	water (Brønsted-Lowry acid)	⇋	conjugate acid of the base	+	conjugate base of water
   NH3(aq)	+	H2O(l)	⇋	NH4+(aq)	+	OH-(aq)
   CH3NH2(aq)	+	H2O(l)	⇋	CH3NH3+(aq)	+	OH-(aq)
   OH-(aq)	+	H2O(l)	⇋	H2O(l)	+	OH-(aq)

   Note: we are indicating that the proton-transfer reaction occurs in the solvent water by including the states of matter in round brackets:

   ⚛ (l) indicates that water is present as a liquid

   ⚛ (aq) indicates that the species is present in aqueous solution (that is, water is the solvent)

Brønsted-Lowry Proton-Transfer Reactions in Aqueous Solutions : Neutralisation Reactions

Neutralisation (or neutralization) is the name given to a reaction between an Arrhenius acid and an Arrhenius base which produces a salt and water as shown in the equations below:

Let's apply the Brønsted-Lowry theory of acids and bases to this type of chemical reaction.

First, Arrhenius defined an acid as a species that dissociates in water to produce hydrogen ions (H⁺_(aq)) and that a base dissociates in water to produce hydroxide ions (OH^-_(aq)):

Therefore the species which react in aqueous solution are the ions:

H⁺_(aq) + A^-_(aq) + M⁺_(aq) + OH^-_(aq) → H₂O_(l) + M⁺_(aq) + A^-_(aq)

Since M⁺_(aq) and A^-_(aq) exist on both the left hand side and the right hand side of the equation, they do not take part in the reaction, they are spectator ions, so we can ignore them.
This allows us to write the following net ionic equation to represent the neutralisation reaction:

H⁺_(aq) + OH^-_(aq) → H₂O_(l)

We can see that an Arrhenius neutralisation reaction involves the transfer of a proton (H⁺) from the acid to the hydroxide ion (OH^-) of the base to form a water molecule (H₂O).

So, in Brønsted-Lowry terms, an Arrhenius neutralisation reaction can be viewed as a proton-transfer reaction in which the acid dissolves in water to produce H₃O⁺ which then reacts with OH^- to form water molecules as shown in the chemical equations below :

We can therefore write the following chemical equation to represent the proton-transfer reaction involved in an Arrhenius acid-base neutralisation reaction:

OH^-_(aq) + H₃O⁺_(aq) ⇋ 2H₂O_(l)

By applying Brønsted-Lowry theory of acids and bases to an Arrhenius "neutralisation" reaction we would now see it as just another example of a proton-transfer reaction: a proton (H⁺) is transferred from a Brønsted-Lowry acid (H₃O⁺) to a Brønsted-Lowry base (OH^-) producing the conjugate base of the acid (H₂O) and the conjugate acid of the base (also H₂O).

Examples of neutralisation reactions:

But the Brønsted-Lowry theory of acids and bases can be used to describe reactions that are NOT hydrolysis reactions nor neutralisation reactions...

Proton-Transfer Reactions : Non-Arrhenius Aqueous Acid-Base Reactions

Consider amines, general formula R-NH₂.
Amines are examples of non-Arrhenius bases because they are "bases" but they do not dissociate in water to produce hydroxide ions, OH^-.
Amines are examples of Brønsted-Lowry bases because they can accept a proton to form the R-NH₃⁺ ion.
For example, an amine might act as a Brønsted-Lowry base by accepting a proton from a water molecule in a "hydrolysis reaction" as shown below:

The hydrolysis of the amine is a proton-transfer reaction because a proton has been transferred from a water molecule to the amine molecule.

The amine could also act as a Brønsted-Lowry base by accepting a proton from an Arrhenius acid in a "neutralisation reaction".

Examples of the "neutralisation" of an amine by an acid are given below:

Reactions that are NOT Proton-Transfer Reactions⁽⁶⁾

The defining characteristic of a proton-transfer reaction is that at some point in the reaction a hydrogen atom must lose an electron to form a proton, and that this proton is then transferred to a different species.
If a proton is not formed, or if a proton is not transferred to a different species, then the reaction is not a proton-transfer reaction.

Let us now consider some examples of reactions that involve hydrogen atoms, but which are NOT proton-transfer reactions:

• metal + non-oxidising acid → salt + hydrogen gas

  For example: Mg_(s) + HCl_(aq) → MgCl_2(aq) + H_2(g)
  In order for the hydrogen gas to form, HCl is NOT releasing protons (H⁺), HCl must be releasing hydrogen atoms, that is hydrogen atoms that retain the electron:

  H .. :Cl : ..

  →

  . :Cl : ..

  + H.

  Two hydrogen atoms, H. + H., each contribute their electron to form a bonding pair of electrons in a covalent bond, H:H (H_2(g)).
  A chlorine atom, with 7 valence electrons, requires only 1 electron to complete its valence shell electrons, while a magnesium atom has 2 valence electrons which it will readily lose to complete its valence electron shell.

  .. :Cl . ..

  +

  .Mg.

  +

  .. .Cl : ..

  →

  .. :Cl ..- ..

  Mg2+

  .. ..Cl :- ..

  There has been no transfer of protons from a Brønsted-Lowry acid to a Brønsted-Lowry base so this is NOT an example of a proton-transfer reaction.

• Hydrogen halide addition to an unsaturated hydrocarbon (Hydrohalogenation of Alkenes)

  For example, the addition of hydrogen chloride to ethene:

  ethene	+	hydrogen chloride	→	chloroethane
  H2C=CH2	+	HCl	→	CH3CH2Cl

  In order for the carbon atoms to form covalent bonds with the H atom and Cl atom of the HCl, the H must retain its electron, and the Cl atom must maintain its 7 valence electrons.
  When the double bond between carbon atoms breaks, each carbon atom retains one of the bonding pairs of electrons.
  One carbon atom then shares this electron with the Cl atom to form a covalent bond, and the other carbon atom shares its electron with the hydrogen atom to form a covalent bond.
  At no point is a proton transferred from an acid to a base, so this is NOT an example of a proton-transfer reaction.

Worked Example of Proton-Transfer Reaction Question