Halogenoalkanes and Hydrolysis Reactions

Halogenoalkanes

What are halogenoalkanes?

A homologous series of organic compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms.

What is the general formula of a halogenoalkane containing one halogen atom?

CₙH₂ₙ₊₁X, where X represents a halogen atom.

What does X represent in the general formula of a halogenoalkane?

A halogen atom: fluorine, chlorine, bromine or iodine.

What does R represent in organic chemistry?

An alkyl group, such as methyl or ethyl.

How can the formula of a halogenoalkane be simplified using R?

RX.

Can a halogenoalkane contain more than one halogen atom?

Yes, a molecule can contain two or more halogen atoms.

How are halogen substituents represented when naming halogenoalkanes?

Using the prefixes fluoro-, chloro-, bromo- and iodo-.

How are halogenoalkanes numbered when naming them?

Number the longest carbon chain so that the substituents have the lowest possible numbers.

In what order are different halogen substituents written in a halogenoalkane name?

Alphabetical order.

What is the name of CH₂Cl–CHCl–CH₃?

1,2-dichloropropane.

What is the name of CH₂Br–CH₂–CH₂Cl?

1-bromo-3-chloropropane.

What is the name of CCl₄?

Tetrachloromethane.

What is the name of CH₃–CHF–CH₂–CH₃?

2-fluorobutane.

How are halogenoalkanes classified as primary, secondary or tertiary?

By the number of alkyl groups attached to the carbon atom bonded to the halogen.

What is a primary halogenoalkane?

A halogenoalkane in which the carbon bonded to the halogen is attached to one alkyl group.

What is the general abbreviated formula of a primary halogenoalkane?

RCH₂X.

What is a secondary halogenoalkane?

A halogenoalkane in which the carbon bonded to the halogen is attached to two alkyl groups.

What is the general abbreviated formula of a secondary halogenoalkane?

R₂CHX.

What is a tertiary halogenoalkane?

A halogenoalkane in which the carbon bonded to the halogen is attached to three alkyl groups.

What is the general abbreviated formula of a tertiary halogenoalkane?

R₃CX.

Why is the C–X bond in a halogenoalkane polar?

The halogen is more electronegative than carbon, so it attracts the bonding pair of electrons more strongly.

What partial charge does the carbon in a C–X bond have?

δ+.

What partial charge does the halogen in a C–X bond have?

δ−.

How does the electronegativity of the halogens change down Group 7?

It decreases from fluorine to iodine.

How does the polarity of the C–X bond change from fluoroalkanes to iodoalkanes?

It decreases because the electronegativity of the halogen decreases down the group.

Why is the carbon bonded to the halogen susceptible to attack?

It is δ+ and therefore electron deficient.

What is a nucleophile?

A species that donates a lone pair of electrons to form a covalent bond with an electron-deficient atom.

What types of species can act as nucleophiles?

Negative ions or molecules containing a slightly negative atom with a lone pair of electrons.

What attracts a nucleophile to a halogenoalkane?

The δ+ electron-deficient carbon atom bonded to the halogen.

What electron pair does a nucleophile use when attacking another species?

A lone pair of electrons.

Why are nucleophiles more strongly attracted to fluoroalkanes than chloroalkanes?

The C–F bond is more polar than the C–Cl bond, so the carbon atom has a greater δ+ charge.

What is a hydrolysis reaction?

A reaction in which water or hydroxide ions replace an atom in a molecule with an –OH group.

What happens when a halogenoalkane undergoes hydrolysis?

The halogen atom is replaced by an –OH group, producing an alcohol.

What is the general equation for hydrolysis of a halogenoalkane with water?

RX + H₂O → ROH + HX.

What is the ionic equation for hydrolysis of a halogenoalkane?

RX + H₂O → ROH + H⁺ + X⁻.

What does ROH represent in a halogenoalkane hydrolysis reaction?

The alcohol produced.

What happens to the C–X bond during hydrolysis?

The C–X bond breaks and the halogen leaves as X⁻.

Why is the reaction with water called hydrolysis?

“Hydro” refers to water and “lysis” means splitting.