Substitution vs Elimination on halides

In summary, the type of base used in a reaction with a halide can greatly affect the resulting product. A strong base will typically lead to an elimination reaction, producing mostly alkenes. On the other hand, a weak base is more likely to cause a substitution reaction, with the product being a substituted halide. This is due to the stability of the carbocation intermediate, which is formed during the substitution reaction. Primary halides, which have fewer alkyl groups attached to the carbon, produce less stable carbocations and are therefore more likely to undergo substitution. Additionally, the reactivity order for substitutions is I>Br>Cl>F, with the type of leaving group playing a significant role in this trend.
  • #1
jaumzaum
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If you add a strong base to a halide, you get mostly the alkene. If you add a weak base, especialy on primary halides not branched on the β carbon, the product is mostly the substituted. Why is that?

1) The mechanism for the substitution reaction is the heterotytic break of the C-X (where X is the halide) bond, producing the carbocation and X-.
The carbocation, though, is more stable as more alkyl groups are added to the α (or β) carbon. In other words, terciary halides and secondary halides produce a much more stable carbocation than primary halides. So why is the primary the only one that can be feasibly substituted?

2) Why a stronger base facilitates the elimination?

3) The reactivity order on substitutions is I>Br>Cl>F. Why?
 
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  • #2
1) they are substituted in an Sn2 reaction because the carbocation is unstable and will not stay there, it needs to have an attachment to be stable

2) bases abstract hydrogens, whereas nucleophiles do an attack on the carbon. You need to understand the difference between the two

3) Look at leaving group ability and understand what a leaving group is.
 
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1. What is the difference between substitution and elimination on halides?

Substitution and elimination are two different types of reactions that can occur on halides, which are compounds containing a carbon-halogen bond. In substitution, the halogen atom is replaced by another group or atom, while in elimination, the halogen atom is removed entirely from the molecule.

2. How do substitution and elimination reactions differ in terms of mechanism?

The mechanism for substitution involves a nucleophile attacking the carbon-halogen bond and displacing the halide, resulting in a new compound. In elimination, a base removes a proton from the carbon adjacent to the halogen, resulting in the formation of a double bond and the elimination of the halide ion.

3. Which type of reaction is more likely to occur on primary halides?

Substitution is more likely to occur on primary halides, as they are less sterically hindered and the carbon-halogen bond is more easily broken. Elimination is more likely to occur on tertiary halides, as they are more sterically hindered and the carbon-halogen bond is more difficult to break.

4. Can a single halide molecule undergo both substitution and elimination reactions?

Yes, a single halide molecule can potentially undergo both substitution and elimination reactions. The likelihood of each reaction occurring depends on the specific conditions and reactants present.

5. How do I determine which type of reaction will occur on a specific halide compound?

The type of reaction that will occur on a specific halide compound depends on a variety of factors, including the nature of the halide, the strength of the nucleophile or base present, and the reaction conditions. It is important to consider these factors and consult a reaction mechanism chart to determine the most likely outcome.

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