ch13 Flashcards
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
Optical isomerism
When is cis-trans isomerism applicable?
Molecules have a C=C double bond
Each carbon in the double bond is attached to two different groups.
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
Outline an electrophilic addition reaction between an alkene and hydrogen bromide
mechanism with H-Br is simpler as H-Br is already polar. The H has the partial positive charge
(δ+) so will attract the electrons from the pi-bond ( δ
+ H is the electrophile).
a bromide ion is formed which will bond to the positively charged carbon in the carbocation. H-Br has been added across the former C=C double bond.
Reactions between alkenes and water or
H2 will follow a similar mechanism.
How does electrophilic addition take place with molecules that are non polar?
As a non-polar Br2 molecule approaches an alkene the pi-electrons in the double
bond repel the bonding electrons in the Br2 creating an induced dipole.
The delta positive Br atom attracts electrons from the pi-bond causing the pi-
bond to break. (delta positive Br atom is the electrophile).A new bond forms from the C in the alkene to the Br atom causing the Br-Br
bond to break (via heterolytic fission). The formerly shared pair of electrons
becomes a lone pair on the delta-negative bromine atom converting it to a fully
negative bromide ion.
The species with a positively charged carbon atom is called a carbocation.
The bromide ion uses a lone pair to form a new covalent
bond to the positively charged carbon.
This completes the addition of Br2 across the former C=C
double bond.
What is Markownikoff’s rule?
The more electron density a positively charged
carbon can receive, the more stable it will become.
Why is the tertiary carbocation the most stable?
the more alkyl groups attached to positively charged carbon, the more charge is spread out, making the ion more stable
What is the name of the reaction for a polymers formation from alkenes?
Addition polymerisation
How is the monomer unit displayed?
small molecule that combines with many other monomers to form a polymer.
How is the repeat unit of the polymer displayed?
in square brackets always.
After bracket put letter n to show there is a large number of repeats.
What are the conditions for a industrial polymerisation reaction?
high temp and high pressure using catalysts
