Alkenes Flashcards
What are alkenes
unsaturated hydrocarbons
What types of bonds are involved in an alkene double bond
1 pi bond and 1 sigma bond
What is a sigma bond
formed between 2 atoms where 2 orbitals overlap each other
what is a pi bond
formed by the overlap of 2 p-orbitals
where is the pi bond electron density situated
above and below the line where the sigma bond is formed
why are alkenes unable to rotate around the double bond
pi bonds lock the carbon atoms in place
What angle is formed by a carbon with double bond
120 degrees
what causes a 120 degree angle to form between angles in an alkene around a carbon with a double bond
There are free bonds around the carbon meaning there are 3 regions of repulsion causing a 120 degree angle
What is stereoisomerism
same structural formula, different arrangement of atoms in space
What is E/Z isomerism
When rotation of atoms is restricted due to pi bonds leading to a fixed position
Where does EZ isomerism occur
in double bonded compounds where carbons are attached either side of the bond
Cis-Trans isomerism is what
when the carbon atoms are attached to methyl groups that are either opposite or the same side
What is more reactive, alkanes or alkanes
alkenes
what makes alkenes reactive
pi bonds, electrons are exposed to atoms causing it to break the double bond
What makes pi bonds weaker than sigma bonds
bond enthalpy
What type of reactions do alkenes undergo
addition reactions
what happens in an addition reaction
a molecule will break the double bond of another molecule to allow the 2 reactants to form 1 product
How does the hydrogenation of an alkene work
hydrogen gas reacts with the double bond of an alkene with the help of a nickel catalyst to form an alkane
How does the halogenation of an alkene work
the hydrogen halide reacts with an alkene to break the alkene into an alkane with a carbocation where the hydrogen will react to help form a haloalkane
What test can be used to test for unsaturated molecules
bromine water can be added
what colour change is seen between bromine water if a C=C bond is present
orange —> colourless
What is an electrophile
an electron pair acceptor
What mechanism is seen in the reaction of an alkene to an alkane
electrophilic addition
What mechanism is seen in the reaction of an alkene to an haloalkane
electrophilic addition
What does a double bond represent
A region of high electron density due to pi bonds
Describe the electrophilic addition of ethene with hydrogen gas
- ethene double bond exposed to hydrogen gas forming a carbocation of ethane and hydrogen -1 ion
- The hydrogen -1 ion will react with the carbocation double bond which will form ethane
What partial charges are formed in H-Br
H has delta + charge
Br has delta - charge
How are partial charges formed in a Br-Br molecule
The Br-Br molecule will come close to the double bond which causes the electrons in the Br-Br molecule to repel leading to induced dipoles forming where the has a positive and negative partial charge on each atom
What type of bond fission occurs when a molecule reacts with a C=C bond
heterolytic fission
What is a primary carbocation
when the carbocation is attached to 1 carbon
what is a secondary carbocation
when a carbocation is attached to 2 carbons
what is a tertiary carbocation
when a carbocation is attached to 3 carbons
How can alkyl group be represented
-R
What type of carbocation is the most stable
tertiary
What type of carbocation is the least stable
primary
what makes tertiary carbocations more stable than any other carbocation
the 3 carbon atoms push electrons into the carbon making it more positively charged leading to a more stable carbocation
