Alkenes Flashcards
(19 cards)
What is the general formula for alkenes?
CnH2n (for aliphatic hydrocarbons).
What is the structure and bond angle for alkenes?
Planar, bond angle 120 degrees.
What are the two parts to a double bond?
The sigma bond and a pi bond.
How does a double bond form?
A sigma bond is formed directly between two carbon atoms by the overlap of orbitals. A pi bond is formed above and below the plane of the carbon atoms by sideways overlap of p-orbitals. In each carbon involved in the double bond, 3 of its electrons go into sigma bonds and 1 goes into a pi bond.
When an alkene reacts which bond breaks?
The pi bond breaks.
Conditions for the hydrogenation of an alkene?
A hydrogen gas and an alkene is passed over a finely divided nickel catalyst. The conditions are 450k and 1000kPa. This produces an saturated alkane.
Write the equation for the hydrogenation of ethene?
C2H4 + H2 —-> C2H6
Uses of hydrogenation?
Used in margarine manufacture.
Conditions for the halogenation of an alkene?
Alkenes react with halogens at room temperature. This produces a di-substituted halogenoalkane.
Write the equation for the halogenation of ethene with bromine?
C2H4 + Br2 —-> C2H4Br2
Conditions for the direct hydration of an alkene?
Steam and the gaseous alkene are heated to high temperatures (600k) and high pressures (6MPa) in the presence of a phosphoric acid catalyst. This produces an alcohol.
Write the equation for the direct hydration of ethene?
C2H4 + H2O —-> C2H5OH
Uses of direct hydration?
Used in ethanol manufacture.
Conditions for the electrophilic addition of an alkene?
Electrophiles are attracted to the pi electrons. This occurs at room temperature. A halogenoalkane is formed.
Write the equation for the addition of hydrogen bromide to ethene?
C2H4 + HBr —-> C2H5Br
What is the mechanism of the addition of hydrogen bromide to ethene?
Hydrogen bromide is a polar molecule.
(H+ - Br-). The electron pair in the pi bond is attracted to the slightly positive hydrogen atom, causing the double bond to break. A bond forms between one of the carbon atoms and the hydrogen atom. The H-Br bond breaks by heterolytic fission and Br- is formed and a carbocation is formed. The positively charged carbocation is unstable and quickly reacts with the bromide ion to form the organic product, bromoethane.
Which way do the curly arrows point?
They point in the direction for which the electrons are moving.
Addition of bromine to an alkene?
When the bromine approaches the alkene, the electrons in the pi bond repel the electrons in the Br-Br bond. The Br-Br is now slightly polar. The electron pair in the pi bond is attracted to the slightly positive bromine atom, which causes the double bond to break. The Br-Br bond breaks by heterolytic fission and a Br- and carbocation is formed. The positively charged carbocation is unstable and quickly reacts with the bromide ion to form the organic product.
What to do with polymers?
Recycling - separate out into types.
Combustion - for energy production.
Cracking - Use as a feedstock.
