Alkenes Flashcards
Characteristics of alkanes
C=C double bond
Formula c2h2n
They react in a similar manner
They differ by CH2
Test for alkanes
Bromine water and shake and if it goes colourless then alkanes are present
What is electrophillic addition?
Electrophilic addition is the addition of an electrophile to a double bond
What happens in electrophillic substitution?
The C=C double bond is broken, and a new single bond is formed from each of the two carbon atoms
What do electrophillic addition reactions include?
Electrophilic addition reactions include the addition of:
Steam (H2O (g))
Hydrogen halide (HX)
Halogen
What is a feature of a double bond in alkenes?
The double bond in alkenes is an area of high electron density (there are four electrons found in this double bond)
This makes the double bond susceptible to attack by electrophiles (electron-loving species)
Describe Electrophilic addition of hydrogen bromide
A molecule of hydrogen bromide (HBr) is polar as the hydrogen and bromine atoms have different electronegativities
The bromine atom has a stronger pull on the electrons in the H-Br bond
As a result of this, the Br atom has a partial negative and the H atom a partial positive charge
In an addition reaction with hydrogen bromide what is the electrophile?
In an addition reaction, the H atom acts as an electrophile and accepts a pair of electrons from the C=C bond in the alkene
The H-Br bond breaks heterolytically, forming a Br- ion
This result in the formation of a highly reactive carbocation intermediate which reacts with the Br- (nucleophile)
What type of molecule is a bromine molecule?
Bromine (Br2) is a non-polar molecule as both atoms have similar electronegativities and therefore share equally the electrons in the covalent bond
What happens when a bromine molecule gets close to the double bond in an alkene?
when a bromine molecule gets closer to the double bond of an alkene, the high electron density in the double bond repels the electron pair in Br-Br away from the closest Br atom
As a result of this, the closest Br atom to the double bond is slightly positive and the further Br atom is slightly negatively charged
What does the closest bromine atom act as in an addition reaction?
In an addition reaction, the closest Br atom acts as an electrophile and accepts a pair of electrons from the C-C bond in the alkene
The Br-Br bond breaks heterolytically, forming a Br- ion
This results in the formation of a highly reactive carbocation intermediate which reacts with the Br- (nucleophile
Conditions for electrophillic addition of HBr
Gaseous hydrogen bromide
Conditions for electrophillic addition of bromine
Aqueous bromine
What happens to the groups attached to the C=C bonds?
In unsaturated compounds, the groups attached to the C=C carbons remain fixed in their position
This is because free rotation of the bonds about the C=C bond is not possible due to the presence of a π bond
What are carbocations?
Carbocations are positively charged carbon atoms with only three covalent bonds instead of four
What are the three types of carbocations?
There are three types of carbocations: primary, secondary and tertiary
What are alkyl groups attached to positively charged carbon atoms ?
The alkyl groups attached to the positively charged carbon atoms are ‘electron donating groups’
This is also known as the inductive effect of alkyl groups
How is the inductive effect illustrated?
The inductive effect is illustrated by the use of arrowheads on the bonds
What do alkyl groups do in presence of carbocations?
The alkyl groups push electrons away from themselves towards the positively charged carbon
This causes the carbocation to become less positively charged
As a result of this, the charge is spread around the carbocation which makes it energetically more stable
Which type of carbocations are the most stable ?
tertiary carbocations are the most stable as they have three electron-donating alkyl groups which energetically stabilise the carbocation
What species are carbocations ?
Due to the positive charge on the carbon atom, carbocations are electron-loving species (electrophiles)
