CAPE CHEMISTRY U2 M1 SS 2.6 – 2.7 REACTIONS OF HALOGENOALKANES – PAGE 1 Useful Terms And Concepts Halogenoalkane ‐ This is a compound in which a halogen atom (F, Cl, Br, or I) replaces a hydrogen atom of an alkane. For example CH 3 Cl and CH 3 CH 2 Br are halogenoalkanes. Halogenoalkanes are more commonly called alkyl halides and/or haloalkanes. The functional group of halogenoalkanes is h bl 09 The Inductive Effect Another way of representing the polarity of a bond involving two atoms with different electronegativities is to draw an arrow on the bond in the direction in which the electrons are attracted. So a carbon‐halogen bond could be represented: shown below. Substitution Reaction ‐A substitution reaction occurs when an atom (or group) from an added reagent substitutes for one in the organic reactant: THERN MAIN ROAD CUNUPIA 693290 rather than showing that the halogen draws electrons towards itself. This is sometimes called an inductive effect and halogens are said to have a negative inductive effect. Alkyl groups like methyl‐, ethyl‐, etc. have the opposite effect. They tend to release electrons and have a positive inductive effect, as shown by the Note that the C atom is bonded to the same number of atoms in the product as in the reactant. The C atom may be saturated or unsaturated. X and Y can be many different atoms but generally not C. Hydrolysis ‐ A reaction between a compound and water. The hydroxide ion hydrolyses halogenoalkanes to alcohols. Inductive Effect ‐ The effect in which substituent atoms or groups in an organic compound can attract (‐I or negative inductive effect) or push away electrons (+I or positive inductive effect), forming polar bonds. MISTRY LESSONS 2012 L.P. #307 SOUT direction of the arrow. The effect is increased if more than one alkyl group is attached to the same carbon: Nucleophilic Substitution (S N ) ‐ A reaction involving the substitution of an atom or group of atoms in an organic compound with a nucleophile as the attacking substituent. Transition State (Activated Complex) ‐ Symbol:‡. A short‐lived high‐energy molecule, radical, or ion formed during a reaction between molecules possessing the necessary activation energy. The transition state decomposes at a definite rate to yield either the reactants again or the final products. The transition state can be considered to be at the top of the energy profile. 32909 GLOBAL CAPE CHEM Reaction With OH ‐ ‐ Alcohol Formation Halogenoalkanes provide one of the most useful methods of preparing alcohols. Halogenoalkanes undergo nucleophilic substitution reactions , in which a nucleophile displaces the halide leaving group from the Intermediate ‐ A transient species that exists between reactants and products in a state corresponding to a local energy minimum on a potential energy diagram. OUTHERN MAIN ROAD CUNUPIA 69 Classification Of Halogenoalkanes Halogenoalkanes are classified as being primary (1°), secondary (2°), or tertiary (3°). This classification is based on the carbon atom to which the halogen is directly attached . If the carbon atom that bears the halogen is which a nucleophile displaces the halide leaving group from the halogenoalkane substrate. The following is a typical nucleophilic substitution reaction. Bromoethane reacts with the hydroxide ion to give ethanol and the bromide ion. The hydroxide ion is the nucleophile. It reacts with the substrate (bromoethane) and displaces the bromide ion. The bromide ion is called the leaving group. HEMISTRY LESSONS 2012 L.P. #307 SO attached to only one other carbon, the carbon atom is said to be a primary carbon atom and the halogenoalkane is classified as a primary halogenoalkane. If the carbon that bears the halogen is itself attached to two other carbon atoms, then the carbon is a secondary carbon and the halogenoalkane is a secondary halogenoalkane. If the carbon that bears the halogen is attached to three other carbon atoms, then the carbon is a tertiary carbon and the halogenoalkane is a tertiary halogenoalkane. Examples of primary, secondary and tertiary halogenoalkanes are the following: In reactions of this type, one covalent bond is broken, and an new covalent bond is formed. In this example, the carbon‐bromine bond is broken and the carbon‐oxygen bond is formed. The leaving group (bromide) takes with it both of the electrons from the carbon‐bromine bond, and the nucleophile (hydroxide ion) supplies both electrons for the new carbon‐oxygen bond. These ideas are generalized in the following equation for a nucleophilic substitution reaction: GLOBAL CAPE CH For purposes of the CAPE syllabus, Nu = OH ‐ and L = Br or Cl. Choice Of Mechanism Primary halogenoalkanes S N 2 mechanism and tertiary halogenoalkanes S N 1 mechanism. S d hl lk b th t ll lik l t d S 1 S 2 Wh i dt k hi th i f t i Material compiled by Denison at Global in Cunupia. Interested students can call 693‐2909 Secondary halogenoalkanes as can be seen on the next page are equally likely to undergo S N 1 or S N 2. When requiredto makeachoice the primary factor is optical characteristics of the product. Hence, secondary halogenoalkanes ‐ S N 2 if product is optically inverted (i.e. of inverse or reverse chirality compared to the reacting halogenoalkane) and secondary halogenoalkanes ‐ S N 1 if product is racemic (i.e. contains a mixture of the dextro‐ and levorotatory isomers).