Alkenes Flashcards
Define alkene
An unsaturated hydrocarbon with a least 1 carbon to carbon double bond
What is the general formula of an aliphatic alkene
CnH2n
Which alkanes don’t follow the general formula
Cyclic alkenes and alkenes with more than 1 double bond
What are the two types of bonds called in the double bond
Sigma and pi
Where’s the sigma bond in the double bond
Describe the electrons
X
C———C
X
Equal from each carbon atom one above and one the line
Describe the p bond in the double bond
Made from two sideways overlapping p orbitals from each carbon atom each carbon atom contributes 1 electron one pi bond consists of 2 regions one above the molecule and one below. This is why the molecule is flat (planar ) around the bond
Which bond stops rotation
The pi bond
What shape is around the double bond
Trigonal planar
Why is the region around the bond trigonal planar
There are three regions of electron density around each of the carbon atoms
The three regions repel each other as far as possible
All the atoms are in the same plane
What is the bond angle around a double bond
120 degrees
What are stereoisomers
They have the same structural formula but a different arrangement of atoms in space
How does stereoisomerism arise
Rotation around the double bond is restricted and the groups attached to each carbon atom are therefore fixed relative to each other
What are the conditions for E/Z isomerism
A c=c bond
Attached to different groups (has to be one of a group either side of the bond)
Which one (E or Z) is for when the groups are on the same side of the double bond
Z (Zame side)
E Ecross
Which group gets priority to decide if it’s a E or Z isomer
The highest atomic number on each side of the double bond
Do E/Z isomers have the same or different chemical properties and why?
The same because they have the same functional group
What’s the difference between e/z isomers and cis/trans isomers
Cis/trans has to have hydrogen
What is needed for hydrogenation of alkenes
Nickel catalyst
150 degrees
H2 under pressure
What are the conditions in halogenation
Rtp
What are the conditions of halogenation with a hydrogen halide
Rtp
Concentrated Hx
What is Markovnikoff’s rule
When a molecule of Hx reacts with a non symmetrical alkene the major product is the one in which the H-atom attaches itself to the carbon atom already carrying a larger number of hydrogens
Which carbocation is the most stable and why
A) primary
B) secondary
C) tertiary
C) tertiary because each alkyl group donates and pushes electrons towards the positive charge of the carbocation. The positive charge is spread over the alkyl groups . The more alkyl groups attached to the positively charged carbon atom, the more the charge is spread out, making the ion more stable
What is the mechanism called for alkenes
Electrophilic addition
What is an electrophile
An electron pair acceptor
Why does the double bond in alkenes have a high electron density
Because of the presence of pi electrons
What is attracted to the high electron density
Electrophiles
