Alkenes Flashcards
(35 cards)
Are alkenes saturated or unsaturated?
Unsaturated
What is the generic formula of alkenes?
CnH2n
Do cyclic alkenes and alkenes with more than one double bond have the generic formula of CnH2n?
No
Describe the formation of a pi-bond.
One electron from each carbon atom is in a p-orbital. When two p-orbitals overlap sideways, one from each carbon atom of the double bond, a pi-bond is formed.
Describe the pi-bond electron density.
It is concentrated above and below the line joining the nuclei of the bonding atoms.
How does the pi-bond affect the geometry of alkenes?
The pi-bond locks the two carbon atoms in position and prevents them from rotating around the double bond.
Explain why the shape around each of the carbon atoms in the double bond is trigonal planar.
There are areas of electron density around each of the carbon atoms. The three regions repel each other as far apart as possible so the bond angle around each carbon atom is 120. All of the atoms are in the same plane.
What is stereoisomerism?
When molecules have the same structural formula but a different arrangement of the atoms in space.
When can E/Z isomerism occur?
When there is a C=C double bond.
Why does stereoisomerism occur?
It arises because rotation around the double bond is restricted and the groups attached to each carbon atom are therefore fixed relative to each other.
What two things must a molecule have for it to be able to display E/Z isomerism?
A C=C double bond and different groups attached to each carbon atom of the double bond.
What is cis-trans isomerism?
E/Z isomerism for H molecules. There must be a hydrogen group on each carbon.
What is the relationship between cis-trans isomerism and E/Z isomerism?
The cis isomer is the Z isomer and the trans isomer is the E isomer.
How do you know which group is the priority group when naming molecules?
The higher the atomic number, the higher priority.
What happens if the two atoms attached to a carbon atom are the same?
You have to look for the first point of difference.
Give four addition reactions of alkenes.
Hydrogen (in the presence of a nickel catalyst, halogens, hydrogen halides, steam (in the presence of a nickel catalyst)
Describe hydrogenation reactions of alkenes.
When an alkene such as propene is mixed with hydrogen and passed over a nickel catalyst at 423K, an addition reaction takes place to form an alkane.
Describe halogenation reactions of alkenes.
Alkenes undergo a rapid reaction with the halogens chlorine or bromine at room temperature.
Describe what bromine water is used for and the process involved.
Can identify if there is a C=C bond present (unsaturation). The bromine water will go from orange to colourless.
Describe the reactions of alkenes with hydrogen halides.
Alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes. If the alkene is a gas they can just be mixed if not it must be bubbled through.
Describe hydration reactions of alkenes.
Alcohols are formed when alkenes react with steam in the presence of a phosphoric acid catalyst.
What is the reaction mechanism for addition reactions with alkenes?
Electrophilic addition
What is an electrophile?
An atom or group of atoms that is attracted to an electron rich centre and accepts an electron pair.
What is Markownikoff’s rule?
When a hydrogen halide reacts with an unsymmetrical alkene the hydrogen hailed attaches itself to the carbon atom of the alkene with the greater number of hydrogen atoms and the smaller number of carbon atoms.
