Chapter 14- Alkenes Flashcards
What are alkenes?
Unsaturated hydrocarbons containing a C=C double bond
What is the general formula for alkenes?
CnH2n
What is a double covalent bond made up of and what does this make it?
Central sigma bond (single covalent bond) with two parts of the pi bond on either side.
It is an area of high electron density making it susceptible to attack from electrophiles.
Why can a double bond not rotate?
The pi bond causes restricted rotation
What is the test for the C=C double bond and other unsaturated compounds?
Bromine water turns the solution from orange-brown to colourless if a double bond is present in the substance as the bromine is added across the C=C double bond
What are electrophiles and what are they attracted to?
Species that accept a pair of electrons. They are attracted to areas of high electron density/ negative charge.
What are some common electrophiles?
HBr (hydrogen halides)
Br2
H2SO4
What is the outline of the mechanism for an alkene reacting with a hydrogen halide?
-Halide is more electronegative than hydrogen so the molecule is polar, H being δ+
-H δ+ is attracted to C=C bond due to high electron density
-A pair of electrons from C=C bond forms bond with H δ+ to form a positive carbocation and both electrons in HBr bond move to Br δ- to form :Br- ion
-Br- ion attaches to the positive carbocation forming a bond with it
-Remember Markovnikov’s rule
What is the outline of the mechanism for an alkene reacting with halogens?
-Halogen has a temporary dipole so δ+ end is attracted to C=C bond, an area of high electron density
-Electrons in C=C double bond are attracted to δ+ pole, repelling the electrons in the halogen bond, strengthening the dipole of the molecule
-Two electrons from C=C bond form a bond with δ+ atom while other leaves as a halide ion
-Halide ion attaches to the positive carbocation forming a bond with it
What is the outline of the mechanism for an alkene reacting with concentrated sulfuric acid?
-H δ+ in sulfuric acid (H-O-SO3H) is attracted to C=C double bond due to high electron density
-Two electrons from C=C bond move to H δ+ to form a bond and electrons in H- O bond move to O δ-
-Negatively charged hydrogensulfate ion attaches itself to positive carbocation
What happens when water is added to a reaction between an alkene and concentrated sulfuric acid?
When water is added to the product, an alcohol is formed and sulfuric acid reforms.
The overall effect is adding H-OH across the double bond, producing an alcohol and sulfuric acid acts as a catalyst for the process.
How do carbocations vary?
Carbocations vary in stability which is determined by the number of alkyl (R) groups bonded to the positive carbocation.
The least stable is a primary carbocation, with one alkyl group, and the most stable is a tertiary carbocation with three alkyl groups.
Why do major and minor products form?
Major and minor products form from electrophilic addition due to relative stability of carbocation intermediates.
The major product is formed from the most stable carbocation while the minor product is formed from the least stable carbocation. This is because the more stable a carbocation is, the more likely it is to form.
What is Markovnikov’s rule?
When hydrogen halides add on to alkenes, the hydrogen adds on to the carbon atom with the most hydrogens.
What are addition polymers?
Polymers formed from monomers with C=C bonds by addition polymerisation
