Alkenes Flashcards
Alkene
Unsaturated hydrocarbons
Contain carbon carbon double bonds
General formula CnH2n
Nomenclature: similar for ____
Alkene nomenclature 1.
- Identify the longest chain that includes both carbons of the double bond and draw a line through it
Alkene nomenclature 2.
- Number the chain from the end nearest the double bond so that the carbon atoms in that bond have the lowest possible numbers
Alkane nonmenclature 3.
If the double bond is the same distance from both ends of the chain, number the chain from the end nearest the first branch point
Alkane nonmenclature 4
Indicate the position of the multiple bond using the lower numbered carbon atom of that bond
Alkene nonmenclature 5
If there is more than one double bond, number the chain from the end nearest the first multiple bond
The chemical reactions of alkenes
Alkenes contain a ‘weaker’ pi bond and so reagents can be added across/to the double bond
Alkenes are much more stronger than alkanes
The most common reaction of alkenes
Electrophilic addition
Definition of addition reaction
Reactions in which two or more atoms/groups/molecules combine to form larger molecules
Example of addition reaction
see in notes from Feb 13 slide number 9
When an unsaturated Alkene reacts with an electrophilic reagent (eg HCl) it gives
A saturated product
Reaction mechanism for the electrophilic addition of HCl to Ethene
The mechanism has 2 steps
It uses curly arrows to show the movement of a pair of electrons
Example of using curly arrows
See notes from Feb 13th, slide number 13 (last slide)
Curly arrows are the notation used by organic chemists to
Indicate the movement of a pair of electrons when bonds are made, broken, or moved
Curly arrow example (bond breaking)
Read from left to right
Start with products A and B
A has a positive charge and B has a negative charge
Starting material A
A—B -> (reaction arrow) A+ + B-
Product at the end
Electrons that form the A—B bond are leaving A and becoming the exclusive property of B (since B is negative it clearly has extra electrons)
Curly arrow example (bond making)
A- + B+ -> A—B
Electrons move from A to B
(Remember an electron pair that was the exclusive property of A is now shared in the bond formed between A and B)
Always wrote curly arrows
On the left side (starting material)
reaction mechanism for the electrophilic addition of HCl to ethene (Step 1)
Step 1: In this step, the partially positive proton of Hal is an electrophile and is attacked by the double bond of ethene which is acting as a nucleophile
Electrophile
Electron lover
It is a species that is attracted to negative charges, often carries a positive or partial positive charge `
Nucleophile
Nucleus lover
It is a species that is attracted to positive charges. It may carry a negative charge or may be an area of electron density such as a double bond
The products of step 1
Carbocation and anion
Carbocation
A carbon atom with a positive charge (note that it only has three bonds) it is very reactive as it only has 6 electrons (rather than the usual 8) in its outer shell and therefore it rapidly combines with the chlorine anion (which supplies the carbocation with 2 electrons)
reaction mechanism for the electrophilic addition of HCl to ethene (Step 2)
The anion and carbocation combine to form a saturated product
Types of alkenes
If an alkene has 2 different substituents at each end, these substituents can be on the same side or opposite side of the double bond
Cis alkene, Trans alkene
Cis alkene
If the two substituents are on the same side of the C=C
Trans alkene
If the two substituents are on opposite side of the C=C
Electrophilic addition to substituted alkenes
So far, we have only considered electrophilic addition to ethene
We need to consider alkenes that have different substituents because there is an issue of regioselectivity
Markovnikov rule
On addition of HX to an alkene, H attaches to the carbon with fewest alkyl groups and X attaches to the carbon with most alkyl groups
We can explain the Markovnikov rule by considering the carbocation intermediates
Carbocations
Carbocations may be classified as primary (1 degree), secondary (2 degrees) or tertiary (3 degrees)
Primary
1 carbon attached
Secondary
2 carbons attached
Tertiary
3 carbons attached
a methyl carbocation
when there are no carbons attached
The order of stability
Tertiary > Secondary > Primary > Methyl
