halogenoalkanes , reactivity ect ....



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  • Halogenoalkanes are compounds in which a halogen atom has been replaced by one of the hydrogen atoms in the alkane.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

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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. 


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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.


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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.
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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 
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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.
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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.


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Aisha, for non stick pans, manufacturers use PTFE = polytetrafluoroethylene, but anyway very good notes. i know that will help me later on!!

thnx **

Tilly - Team GR


Very helpful

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