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- Halogenoalkanes are compounds in which a halogen atom has been replaced by one of the hydrogen atoms in the alkane.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