Alkanols and Hydrohalic Acids (HX) Chemistry Tutorial

Key Concepts

• Alkanols belong to a class of organic compounds known as alcohols, compounds that contain the hydroxyl functional group (OH).
• The active site on an alkanol molecule is the hydroxyl (OH) functional group.
• The halide ion (X^-) from a concentrated solution of a strong hydrohalic acid (HX_(aq)) can replace the hydroxyl functional group to produce a haloalkane (alkyl halide or halogenoalkane) in a substitution reaction.⁽¹⁾

  Name, formula and reactivity towards alkanols of strong hydrohalic acids (2)
  Name	Molecular Formula	strength of acid(3)	Rate of reaction
  hydrochloric acid	HCl(aq)	weaker	slowest
  hydrobromic acid	HBr(aq)	↓	↓
  hydroiodic acid (hydriodic acid)	HI(aq)	stronger	fastest

• The general chemical equation for this substitution reaction is:

  general word equation	alkanol	+	hydrohalic acid	→	haloalkane	+	water
  general molecular equation	R−OH	+	HX	→	R−X	+	H2O

  R is a hydrocarbon chain (an alkane chain which is the parent hydrocarbon).

  X is a halogen atom: chlorine, bromine or iodine.

• Reaction with concentrated HBr and HI : All alkanols (primary, secondary and tertiary) readily react at room temperature with HI or HBr
• Reaction with concentrated HCl :

  ⚛ Tertiary alkanols readily react with HCl and room temperature

  ⚛ Secondary alkanols react with HCl at room temperature in the presence of a suitable catalyst.

  ⚛ Primary alkanols react with HCl only when heated in the presence of a suitable catalyst.

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Alkanol + Hydroiodic Acid

Hydroiodic acid (or hydriodic acid, HI_(aq)) is the strongest of the hydrohalic acids and is the most reactive towards the alkanols.

Any alkanol (R−OH) will readily react with concentrated hydroiodic acid (HI_(aq)) in a substitution reaction.

The iodide ion (I^-) will substitute for the hydroxyl (OH) functional group on the alkanol molecule to produce iodoalkane and water:⁽⁴⁾

For example, the primary alkanol ethanol (CH₃-CH₂OH) will react with concentrated hydroiodic acid (HI_(aq)). The iodide ion (I^-) will replace the hydroxyl (OH) functional group in a substitution reaction to produce iodoethane and water:

The secondary alkanol propan-2-ol will also react with concentrated hydroiodic acid in a substitution reaction producing 2-iodopropane and water:

The tertiary alkanol 2-methylpropan-2-ol, (CH₃)₃COH, will react with concentrated hydroiodic acid, HI_(aq), in a substitution reaction to produce 2-iodo-2-methylpropane and water:

Alkanol + Hydrobromic Acid

Hydrobromic acid, HBr_(aq), is almost as strong as hydroiodic acid, HI_(aq), and therefore is also very reactive towards alkanols.

Any alkanol (R−OH) will readily react with concentrated hydrobromic acid (HBr_(aq)) in a substitution reaction.

The bromide ion (Br^-) will substitute for the hydroxyl (OH) functional group on the alkanol molecule to produce a bromoalkane and water:⁽⁴⁾

For example, the primary alkanol ethanol (CH₃-CH₂OH) will react with concentrated hydrobromic acid (HBr_(aq)). The bromide ion (Br^-) will replace the hydroxyl (OH) functional group in a substitution reaction to produce bromoethane and water:

The secondary alkanol propan-2-ol will also react with concentrated hydrobromic acid in a substitution reaction producing 2-bromopropane and water:

The tertiary alkanol 2-methylpropan-2-ol, (CH₃)₃COH, will react with concentrated hydrobromic acid, HBr_(aq), in a substitution reaction to produce 2-bromo-2-methylpropane and water:

Alkanol + Hydrochloric Acid

Of the strong hydrohalic acids, hydrochloric acid (HCl_(aq)) is the least strong and less reactive towards alkanols.
This means that tertiary alkanols (3° alkanol) will readily react with hydrochloric acid, but the reaction between primary (1°) or secondary (2°) alkanols and concentrated hydrochloric acid requires a suitable catalyst such as anhydrous zinc chloride (ZnCl₂).⁽⁵⁾

Under suitable conditions, an alkanol (R−OH) will react with concentrated hydrochloric acid (HCl_(aq)) in a substitution reaction.

The chloride ion (Cl^-) will substitute for the hydroxyl (OH) functional group on the alkanol molecule to produce a chloroalkane and water:⁽⁴⁾

For example, the primary alkanol ethanol (CH₃-CH₂OH) will react with concentrated hydrochloric acid (HCl_(aq)) in the presence of a ZnCl₂ catalyst and heat. The chloride ion (Cl^-) will replace the hydroxyl (OH) functional group in a substitution reaction to produce chloroethane and water:

The secondary alkanol propan-2-ol will react with concentrated hydrochloric acid in the presence of a zinc chloride catalyst without heating.⁽⁵⁾ The chloride ion (Cl^-) will replace the hydroxyl (OH) functional group in a substitution reaction to produce 2-chloropropane and water:

The tertiary alkanol 2-methylpropan-2-ol will readily react with concentrated hydrochloric acid at room temperatrure (25°C) without the presence of a catalyst. The chloride ion (Cl^-) will replace the hydroxyl (OH) functional group in a substitution reaction to produce 2-chloro-2-methylpropane and water: