DF6: Alkenes- versatile compounds

What is an alkene?

unsaturated hydrocarbon

How are alkenes named?

1. Identify the longest chain and use the appropriate prefix to indicate the number of carbon atoms.

2. Follow the prefix with 'ene' to indicate the compound is an alkene.

3. For alkenes with longer chains or branched chains, indicate the position of the double bond using the lowest number possible.

4. For cyclic alkenes, use the prefix cyclo- (e.g., cyclohexene).

What is the general formula of alkenes?

CnH2n

what are electrophiles?

positive ion or molecule with partial positive charge that is attracted to a negatively charged region and will react by accepting a lone pair of electrons to form a covalent bond

What is electrophilic addition?

reaction where an electrophile reacts with an alkene's electron-rich double bond to form a single larger molecule

Describe the reaction of ethene with bromine.

1. Bromine becomes polarised as electrons are repelled by double bond

2. Bromine atom nearest double bond becomes slightly positive, the other slightly negative

3. Positive bromine atom behaves as an electrophile and reacts with alkene double bond.

4. One of the carbons becomes a carbocation

5. A pair of electrons moves from the negative bromine atom and forms a covalent bond

How many pi/sigma bonds in a single bond?

1 sigma bond

sigma bonds form in

an area of increased electron density

How are sigma bonds formed?

overlap of s orbitals

What kind of bonds in a double bond?

one sigma

one pi

one pi bond consists of ___ areas of negative charge

one ______ one _________

2

above

below

How can you identify an alkene?

brown bromine water becomes colourless

Electrophilic addition of Br₂ to alkene 

Formation of a bromo-alcohol

Conditions for reaction of alkene with hydrogen bromide

room temperature

polar solvent

Conditions for reaction of alkene with water

1. high temperature

2. high pressure

3. catalyst- phosphoric acid adsorbed on solid silica

Mechanisms for reaction of ethene and water

Conditions of reaction of ethene with hydrogen

Platinum catalyst- standard conditions

Finely powdered nickel- high temperature and pressure

Why is a catalyst needed for reaction of alkene and hydrogen?

break strong H-H bonds to form H atoms