Chapter 13 - alkenes Flashcards
What is an alkene?
an alkene is an unsaturated hydrocarbon with the general formula CnH2n
alkenes can also be cyclical CnH2n-2
what is a sigma bond
the overlap of orbitals (usually s orbitals) between adjacent atoms to form a shared pair of electrons with an electrostatic attraction to the nuclei of the bonded atoms
what is a pi bond
the sideways overlap of P orbitals between adjacent atoms to form a shared pair of electrons with an electrostatic attraction to the nuclei of the bonded atoms
what is a double bond
a double bond is made up of a sigma bond and a pi bond
where does the pi bond occur
the pi bond occurs either above or below the sigma bond but is constantly changing
which bond is weaker and why
the pi bond is weaker than the sigma bond because its further from the nucleus and thus the electrons experience less electrostatic attraction to the nucleus.
what is a stereoisomer
“a stereoisomer is an isomer that has the same structural formula but a different arrangement in space”
this occurs because no rotation can occur around a double bond
what are the properties of a double bond
bond enthalpy is almost double the bond enthalpy of a single bond
no rotation can occur around a double bond
what is E-Z isomerism
there must be a C-_C double bond and different groups attached to the C’s of the double bond
A C
\ /
C = C A and B must be different
/ \
B D
what is an E isomer
an isomer where the two groups that are the same (or highest priority) are on opposite sides of the molecule
(comes from German entgegen (opposite))
they ‘ent on the same side
what is a Z isomer
an isomer where the two groups that are the same (or highest priority) are on the same side of the molecule
(comes from the German zusammen (together))
they are on Z same side
when is E-Z isomerism Cis-Trans isomerism
when one of the groups on each side is a hydrogen
what is the Cahn-Ingold-Prelog rule and why is it used
it is used to assign priority where any 2 of the 4 groups attatched to the carbon atoms are not the same.
the groups which are thought of as the same are the ones with the highest priority on each side
how does the Cahn-Ingold-Prelog rule work
priority is first assigned based on the atomic number of the groups immediately attached to the carbon, if their atomic number is the same, then you work off of the atomic numbers of the atoms immediately attached to them and continue until there is a difference.
REMEMBER A DOUBLE BOND DOUBLES ATOMIC NUMBER
why are alkenes far more reactive than alkanes
the pi bond can break fairly easily to attach to other atoms and molecules.
what sort of reactions do alkenes take part in
with hydrogen (and a nickel catalyst)
halogens
hydrogen halides
steam (with phosphoric acid catalyst)
hydrogenation of alkenes (occurs by electrophilic addition)
needs fairly high temperature (423K) and a nickel catalyst
if there’s more than one double bond, more than one hydrogenation can occur
H3C H H H H
\ / | | |
C = C + H2 ——> H - C - C - C - H
/ \ | | |
H H H H H
halogenation of alkenes
the same as in hydrogenation occurs but no catalyst or high temperatures are required.
this is also where the test for the bromination test comes from.
bromine attaches across the double bond and the colour disappears
addition reactions of alkenes with hydrogen halides
these can form two different products, the major product and the minor product
the major product tends to be the one where the halogen is in the middle
hydration of alkenes
alcohols are formed when alkenes react with steam in the presence of phosphoric acid catalyst
one of the H’s from the H2O attaches straight to a carbon, the OH then forms an alcohol group
what is the mechanism for addition reactions in alkenes
electrophilic addition
mechanism for hydrogen halides
1) the hydrogen halide is polar so when it approaches the double bond, the hydrogen (an electrophile) is attacked by the nucleophile double bond
2) an electron pair is donated to the hydrogen of the hydrogen halide and the hydrogen halide splits by heterolytic fission
3) you now have a carbocation and a bromide ion, these attract to each other and the bromide ion donates its electrons to the carbocation to form a haloalkane
differences in the mechanism for electrophilic addition using hydrogen or halogens rather than a hydrogen halide
the same as before but instead of naturally polar molecules, the molecules have induced dipoles formed when the molecule approaches the electron dense double bond region
what does Markownikoff’s rule state
“when a hydrogen halide reacts with an asymmetrical alkene the hydrogen of the hydrogen halide will attach itself to the carbon atom of the alkene with the greater number of hydrogens and the smaller number of carbons attached”
