Chapter 13 Flashcards
What is an alkene?
An unsaturated hydrocarbon consisting of at least one C=C bond
What two types of bonds are in a C=C bond?
Pi and sigma bonds
What is a pi bond?
A pi bond is a type of covalent bond formed from the sideways overlap of adjacent p orbitals, above and below the bonding C atoms
What is a special feature of a pi bond?
It has restricted rotation i.e. it does not allow bonds around the C atom to rotate
What is a stereoisomer?
Compounds with the same structural formula but with a different arrangement in space
What are the conditions needed for a molecule to have E/Z isomerism?
A C=C bond
Different groups attached to each carbon atom of the double bond
How does stereoisomerism arise?
Stereoisomerism around double bonds arises because rotation about the double bond is restricted and the groups attached to each carbon atom are therefore fixed relative to each other.
What is cis-trans isomerism?
A special case of E/Z isomerism in which there is one Hydrogen atom on each side of the carbon-carbon double bond.
What are the conditions needed for a molecule to have cis/trans isomerism?
A C=C bond
Different groups attached to each carbon atom of the double bond
One of the attached groups on each carbon atom of the double bond must be hydrogen
Where there is a hydrogen on each of the double bonded carbon atoms in cis-trans isomerism:
The cis isomer is the Z isomer
The trans isomer is the E isomer
What are the Cahn-Ingold-Prelog rules for naming isomers?
If the groups of higher priority are on the same side of the double bond, the compound is a Z isomer
If the groups of higher priority are diagonally placed across the double bond, the compound is an E isomer
How to assign priority to molecules attached to the C=C bond when deducing the type of isomer (E/Z isomer)?
Examine the atoms attached directly to the carbon atoms of the double bond and see which atom has the higher atomic number. Higher atomic number = higher priority.
If the two atoms attached to a carbon atom in the double bond are the same, then you will need to find the first point of difference. The group which has the higher atomic number at the first point of difference is given the higher priority.
Why are alkenes so reactive?
Alkenes are much more reactive than alkanes because of the presence of the pi bond. Being on the outside of the double bond, the pi electrons are more exposed than the electrons in the sigma bond, therefore a pi bond readily breaks and alkenes undergo addition reactions relatively easily.
The pi bond also readily breaks due to its lower bond enthalpy compared to a sigma bond.
What are the different types of addition reactions that alkenes undergo?
With hydrogen in the presence of a nickel catalyst
With halogens
With hydrogen halides
With steam in the presence of an acid catalyst
Electrophilic addition reactions
What is the hydrogenation of alkenes?
When an alkene is mixed with hydrogen and passed over a nickel catalyst at 423K, an addition reaction takes place to form an alkane.
What is the halogenation of alkenes?
When alkenes undergo a rapid addition reaction with a halogen at room temperature to form a dihaloalkane.
What is the addition reaction of alkenes with hydrogen halides?
Alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes. if the alkene is a gas, then the reaction takes place when the two gases are mixed. If the alkene is a liquid, then the hydrogen halide is bubbled through it.
What is the hydration of alkenes?
When alkenes react with steam in the presence of a phosphoric acid catalyst, H3PO4, alcohols are formed
Describe the electrophilic addition reaction between but-2-ene and hydrogen bromide:
- Bromine is more electronegative than hydrogen, so hydrogen bromide is polar. Hydrogen has a positive dipole and bromine has a negative dipole.
- The electron pair in the pi bond is attracted to the partially positive hydrogen atom, causing the double bond to break
- A bond forms between the hydrogen atom of the H-Br molecule and a carbon atom that was part of the double bond.
- The H-Br bond breaks my heterolytic fission, with the electron pair going to the bromine atom
- A bromide ion Br- and a carbocation are formed.
- In the final step, the Br- ion reacts with the carbocation to form the addition product
What is carbocation stability?
Carbocation stability is linked to the electron donating ability of alkyl groups. Each alkyl group donates and pushes electrons towards the positive charge of the carbocation. 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 addition polymerisation?
When many unsaturated alkene molecules join together to form long saturated chains containing no double bonds
