9 - Alkenes Flashcards
What is an alkene?
An unsaturated hydrocarbon
it has a double bond
What is general formula of an alkene?
C~n H~2n
What is the functional group of alkenes?
C=C
What type of reactions do alkenes take part in?
Addition reactions
How can you test to see if a substance is an alkene?
You react the substance in bromine water
If it is an alkene, the bromine water will turn from brown to colourless
What is the general formula of cycloalkenes?
C~n H~(2n-2)
What is the general formula of arenes?
C~n H~n
What is an example of an arene?
Benzene
How do you need to draw alkenes as displayed formulas?
All bond angles have to be at their maximum
No matter what it ends up looking like
A double bond has the same repulsion as a single bond
What does the number in the middle of an alkene mean?
The carbon which the double bond is on
E.g. but-2-ene:
Double bond is between carbon 2 and carbon 3
What does diene mean?
When there are 2 double bonds
There will be 2 numbers before
What is a double bond made from?
The first C-C bond is a sigma-bond
The second part is a pi-bond
What are valence electrons?
Outer electrons used in bonding
In what 3 ways are sigma-bonds formed?
When 2 S-orbitals overlap
When an S and a P overlap
When 2 P-orbitals overlap
Generally, when do sigma-bonds form?
When there is a direct overlap of atomic orbitals
How is the pi-bond of a double bond formed?
The P-orbitals around the carbons which have a sigma-bond overlap
This overlap is the pi-bond
in short: The pi-bond is caused by sideways overlap of P-orbitals
What effect does the pi-bond have on alkenes?
The pi-bond prevents rotation of atoms around the double bonded carbons
What are stereoisomers?
Molecules with the same structural formula but different arrangements of atoms in space
What are the 2 types of stereoisomers?
E-isomers (trans)
Z-isomers (cis)
What are E-isomers and Z-isomers?
E-isomers:
When the 2 largest groups which are bonded to a central carbon are on the opposite sides of the molecule
Z-isomers:
When the largest groups are on the same side
How do you show if an alkene etc is an E or Z isomer in the name?
Just put E- or Z- before the name
Like E-but-2-ene
What 2 things have to be true for a substance to have stereoisomers?
Double bond (so that there is no rotation)
Each carbon in the double bond is bonded to 2 different atoms or groups
What are Cis and Trans isomers?
A Cis isomer is the Z isomer with a hydrogen on each double bonded carbon
A Trans isomer is the E isomer with a hydrogen in each double bonded carbon
What do you do when classifying stereoisomers in very complex organics?
use the C-I-P rules:
If the 2 highest priority are on the same side, it’s Z and the opposite for E
The priority of an atom is higher if it’s atomic mass is greater
You have to compare the different parts of the structural formulas until one has a higher priority part than another
How reactive are alkenes compared to alkanes and why?
alkenes are more reactive
The pi-bond can break and more sigma-bonds can form
How do pi and sigma bonds compare in terms of strength?
Sigma-bonds are stronger (higher bond enthalpy) than pi-bonds
How do you test for alkenes and cycloalkenes?
Add bromine water
If it is an alkene or cycloalkene
The bromine water will turn from orange to colourless