Halogenoalkane Reactions Section 10.5

Introduction In halogenoalkanes, the carbon atom is partially positive because of the polarity of the C-halogen bond Nucleophile: a species (molecule or anion) which contains a lone pair of electrons that can be donated to an electron-deficient center in an organic molecule to form a dative bond

More The carbon atom in a halogenoalkane is described as being electron deficient This makes halogenoalkanes more reactive than regular alkanes Halogenoalkanes are saturated and will undergo substitution reactions

Nucleophile Substitution The halogen atom is replaced by another atom or group of atoms The hydroxide ion is a nucleophile (OH - ) The OH - will substitute for (replace) the halogen

Mechanism for Nucleophilic Substitution A mechanism is a model of how a reaction occurs through a series of steps The mechanism can be different for primary, secondary, and tertiary halogenoalkanes Curly arrows are used to represent the movement of electron pairs The “head” of the arrow is where the electrons are “headed” or where they end up

Reaction of a Primary Halogenoalkane CH 3 CH 2 Br (aq) + OH - (aq) → CH 3 CH 2 OH (aq) + Br - (aq) This is a single-step reaction The rate expression has been found to be: Rate = k[CH 3 CH 2 Br (aq) ][OH - (aq) ] The reaction is said to be bimolecular Called S N 2 (substitution nucleophilic bimolecular) See the reaction mechanism in notes from class

Important Vocabulary Heterolytic fission: the breaking of a covalent (dative) bond so that one of the atoms or groups separates with both bonding electrons and becomes negatively charged, leaving the other atom or group positively charged Carbocation: an organic ion with a positive charge on an electron deficient carbon atom

Reaction of a Tertiary Halogenoalkane CH 3 C(CH 3 ) 2 Br (aq) + OH - (aq) → CH 3 C(CH 3 ) 2 OH (aq) + Br - (aq) Looks like the same reaction, but the mechanism is different The additional alkyl groups make it difficult for the nucleophilic attack First step is heterolytic fission of the carbon- bromine bond Both electrons in the bond end up on the halogen The carbon atom becomes a carbocation

More about Tertiary See the reaction notes from class. The slow step of this reaction is determined by the concentration of only one reactant (the halogenoalkane) Rate expression: rate = k[CH 3 C(CH 3 ) 2 Br (aq) ] Unimolecular reaction Reaction mechanism is called S N 1

Secondary Halogenoalkanes Mechanism is less easy to define Data show that they undergo a mixture of S N 1 and S N 2, or some mechanism in between the two

More Interesting Stuff Relative reactivity of the different halogens depends on the strength of their bonds with carbon This decreases as we go down the halogen group Iodoalkane has the longest, weakest carbon- halogen bond so it is the most reactive Fluoroalkane is the least reactive