Halogenoalkanes

What are Halogenalkanes 

• Halogenoalkanes are compounds in which a halogen atom has been replaced by one of the hydrogen atoms in the alkane.

• They have single bonds hence the fact they are part of an alkane chain ,they have the formula CnH2n+1X where X resents the halogen atom.

Key points when naming Halogenoalkanes 

• count the longest chain of carbon atoms

• state the position of the halogen atom

•  if there is more than one halogen atom attached  name the compounds listed alphabetically.
• F – fluoro    Cl – Chloro    Br – Bromo   I – Iodo

Reactivity of Halogenoalkanes ?

• Halogenoalkanes have polar carbon – halogen bonds this is due to the electronegativities of the carbon and halogen atoms.

• The halogen atoms are more electronegative compared to the carbon atom therefore the carbon atom is delta + while the halogen atom is delta –  , this is because the electron are being attracted to the halogen atom resulting in a polar bond.

Trend ?

• The polarity of halogenoalkane group deceases down the group because the halogen atoms electronegativity also decrease down group 7.

How do Halogenoalkanes react ?

• The carbon atom is electron defficient and attracts nucleophiles ;an atom or a group of atoms that is attracted to an electron deficient centre , for example H2O , OH- , NH3
• 
• The reaction occurs in a substitution reaction 
• The positive ion or nucleiphile replaces the halogen atom giving two products. 

Hydrolysis of halogenoalkanes ?

• Reaction with water or aqueous hydroxide ions that breaks a chemical compound into two compounds.
• Reacting a halogenoalkane with hot hydroxide ions causes a nucleophilic substitution reaction which will produce an alcohol.
• This production of Alcohols is called hydrolysis.

How does hydrolysis happen ?

• Hydrolysis occurs through a nucleophilic substitution reaction.

• OH- ion has a lone pair of electrons which is attracted to the electron deficient carbon atom  this is also known as nucleophilic attack.

• Donating the lone pair leads to a new covalent bond between the ‘O’ molecule and the carbon atom as a result this causes a break in the carbon – halogen bond by heterolytic fission forming a halide ion.

Rates of reaction of halogenoalkanes

• To find out the rate of reaction of hydrolysis you under go an experiment and calculate the time for the precipitate to form. 

• The halogen alkane is heated with silver nitrate aqueous.
• Ethanol is added and acts as a solvent.
• The water acts as the nucleophile reacting with the halogenoalkane leaving the hailide ions.
• Then the silver nitrate reacts with the hailide ion to form a precipitate .
•  You can identify which halogenoalkane was present ...

• Chloroalkane - AgCl - white precipitate 
• Bormoalkane - AgBr - cream precipitate 
• Idoalkane - AgI - yellow precipitate 

Polarity or Bond enthalpy ?

• Polarity – the stronger the polarity of the carbon – halogen bond the nucleiphile will react faster with the positive carbon atom giving a faster rate of reaction.

• Bond enthalpy - The weaker the bond between the carbon – halogen bond the easier the bond may be broken ,and the carbon atom can from a new bond with the nuclphile .

Bond enthalpy is more important than polarity

• The rate of hydrolysis increase as u descend down the halogen group therefore iodoalkanes reacts the fastest.

Halogenoalkanes in polymers ?

• properties of halogenoalkanes are infuenced by the strength of the carbon - halogen bonds in the long chained polymer.

• Carbon - fluorine bonds are very strong and also make polyer materials resistant to chemical attack , which makes it resistant to heat , electrical insulation and non stick properties for cooking appliances.