Mexhanisms - Alkenes Flashcards
What is electrophilic addition?
alkenes that have a C=C double bond, react with electrophiles, leading to the addition of atoms or groups to each carbon of the double bond.
Why are alkenes susceptible to electrophiles?
m C=C double bond is very electron rich due to the electron cloud of the π bond.
What happens during an addition reaction of alkenes?
In an addition reaction, the C=C double bond opens up and an atom or group of atoms joins onto each carbon of the double bond.
What are some examples of electrophilic addition reactions?
Examples include the addition of steam (H2O(g)), hydrogen halides (HX), and halogens (X2).
What is the product of adding steam to an alkene?
The product is an alcohol when steam is added to an alkene in the presence of a catalyst and heat.
What is the result of adding hydrogen halides (HX) to an alkene?
The result is the formation of a halogenoalkane, with the major and minor products depending on the structure of the alkene.
What occurs when halogens (X2) react with alkenes?
When halogens react with alkenes, they form halogenoalkanes.
What is the polarity of hydrogen bromide (HBr)?
HBr is polar due to different electronegativities of hydrogen and bromine.
What charges do the hydrogen and bromine atoms have in HBr?
The Br atom has a partial negative charge (δ-) and the H atom has a partial positive charge (δ+).
Why does the bromine atom have a stronger pull on the electrons in the H-Br bond?
Bromine has a higher electronegativity than hydrogen, pulling the electrons more strongly towards itself.
What role does the H atom play in the electrophilic addition reaction with HBr?
The H atom acts as an electrophile and accepts a pair of electrons from a bond in the alkene.
What happens to the H-Br bond during the reaction?
The H-Br bond breaks heterolytically, forming a Br- ion.
What is formed as a result of the reaction with HBr?
A highly reactive carbocation intermediate is formed, which then reacts with the Br- ion.
What is the product of the reaction between ethene and HBr?
The product is bromoethane.
Provide an example of an alkene reacting with HBr.
An example is but-2-ene reacting with HBr.
What is the product of the reaction of alkenes with concentrated sulfuric acid?
The product is an alkyl hydrogen sulfate.
This is the first stage of a 2 stage reaction to produce an alcohol from an alkene.
What happens in the second stage of the reaction with alkyl hydrogen sulfate?
The alkyl hydrogen sulfate is warmed with water to produce an alcohol in a hydrolysis reaction.
Hydrolysis is a reaction where a molecule is split by the addition of water.
What role does concentrated sulfuric acid play in electrophilic addition?
Concentrated sulfuric acid adds across the double bond of alkenes and acts as an electrophile.
The hydrogen atom in sulfuric acid has a partial positive charge.
What is formed during the electrophilic addition of alkenes?
A highly reactive carbocation intermediate is formed, which then reacts with the hydrogen sulfate ion.
The H-O bond breaks heterolytically, forming a hydrogensulfate ion, HSO4.
What does water do in the reaction with alkyl hydrogen sulfate?
Water adds across the double bond to form an alcohol and sulfuric acid, with sulfuric acid acting as a catalyst.
What is the role of a carbocation in addition to unsymmetrical alkenes?
Carbocations donate a pair of electrons to an electrophile.
Example: H-C → H-C-C-H
