Chapter 13 - Alkenes Flashcards
The properties of alkenes, Stereoisomerism, Reactions of alkenes, Electrophilic addition in alkenes, Polymerisation in alkenes.
Alkenes are which kind of hydrocarbons?
Unsaturated
What is the general formula for alkenes?
CnH2n
How is a double bond formed?
Three sigma bonds formed. One electron left in a p-orbital. A pi bond forms from the sideways overlap of two p-orbitals.
What prohibits rotation in alkenes?
The pi bond as it locks the two carbon atoms in position.
What is the shape around the carbon atoms in the double bond and why?
Trigonal planar Three regions of electron density Repel each other equally In the same plane 120 degree bond angle
What is stereoisomerism?
The same structural formula but a different arrangement of the atoms in space.
What are the two types of stereoisomerism?
E/Z isomerism - only occurs in compounds with a C==C double bond. Optical isomerism - can occur in a much more wider range of compounds, including alkanes with no functional groups.
When is cis-trans isomerism applicable?
Molecules have a C=C double bond Each carbon in the double bond is attached to two groups that are the same
What is the cis isomer?
Hydrogen atoms on each carbon on the same side.
What is the trans isomer?
Hydrogen atoms diagonally opposite each other.
What are the Cahn-Ingold-Prelog rules for deciding E/Z isomerism?
Higher atomic number is higher priority. Groups of higher priority on the same side of the double bond, then Z isomer.
Describe the hydrogenation of alkenes
An alkene is mixed with hydrogen and passed over a nickel catalyst at 423K forming an alkane.
Describe the halogenation of alkenes
Rapid addition reaction between an alkene and chlorine or bromine at room temperature to form a halogenoalkane.
How are alkenes tested for?
When bromine water (orange) is added to the sample of an alkene, the bromine reacts with the alkene and the colour disappears.
Describe the hydration of alkenes
An alkene is reacted with steam in the presence of a phosphoric acid catalyst forming an alcohol (two products can be formed).
What is the mechanism for addition reactions?
Electrophilic addition