Haloalkanes

These are simply alkanes with a halogen atom bonded to them. They can be classified as primary, secondary or tertiary, like alcohols can be. As you might expect they have different properties to alkanes. Can you explain this and predict which will be the most reactive: a fluoro, chloro, bromo or iodo alkane? What's your logic behind this? Which will have the higher B.P for the same chain length? What's your reasoning?

Look at the following chemguide link. Are you surprised? Can you explain all of it now?

Haloalkanes Introduction

Sustitution reactions

As there is a permanent dipole with the more electronegative halogen leaving the the carbon slighty positive, it is subject to attact by nucleophiles (a lover of positive centres, or better put, an electron pair donator). Therefore, haloakanes can form different substances, depending on what the nucleophile is. Note the nuceophile does not have to have a negative charge, it could be neutral e.g NH3 ammonia, however, it must have a lone pair available to donate. Another question to get you thinking, what factors do you think that the rate of substitution wil depend on? Which haloalkane will have the fastest rate of substitution: fluoro, chloro, bromo or iodo? Which nucleophile will give the fastest reaction: NH3, OH- or CN- ? What's your reasoning? Look at the links below, any surprises?


Substitution with cyanide ions

Substitution with ammonia

Substitution and elimination with hydroxide ions

Substituton mechanism

Elimination reactions

Under different conditions, haloalkanes can undergo elimination reactions forming an alkene. If NaOH is dissolved in hot ethanol, rather than water, then the OH- ion acts a base rather than as a nucleophile and accepts a proton. The type of hoalkane also influences whether elimination or substitution reaction occurs. Tertiary haloalkanes tend to react via elimination and primary via substitution. Can you think of any reasons for this? The following chemguide links are good:

Elimination or substitution

The elimination mechanism

Elimination from unsymetric haloalkanes