alkenes Flashcards
(29 cards)
explain the bonding in an alkene
-The double bond is made up of 2 parts:
* A sigma (σ) bond
* A pi (π) bond
compare the strength of pi bonds compared to sigma bonds
-Pi bond is weaker than sigma bond due to the sideways overlap of orbitals –->requires less energy to
break
what is a pi bond
-pi bond formed by sideways overlap of adjacent p-orbitals above and below the bonding C atom
what is a planar molecule
There is no free rotation about
the C=C bond
describe and explain the shape and bonding in ethene
-trigonal planar
-120 degrees
-central C atom has 4 electron pairs and three regions of electron density
-3 bonding sites
-equal repulsion between 3 bonding sites
what is a high electron density
there is a high
chance of finding an electron and the bond is susceptible to
“opening”
describe and explain the reactivity of alkenes
-very reactive
-because of C=C
-pi bond has high electron density
-pi bond has low enthalpy–> less energy needed to break
why does pi bond have low enthalpy
It is weaker than the
sigma bond as orbitals do not directly overlap and does not
require a lot of energy to break
what is the test for alkenes
-aqueous bromine
-orange solution turns colourless in the prescence of alkenes
-Dihaloalkanes
formed
what forms when hydrogen halides and alkenes react
Haloalkanes
formed
what conditions is needed for the reaction between ethene and hydrogen
-Ni catalyst
-150 degrees
what conditions is needed for the reaction between ethene and steam
-an acid catalyst (H+ ions)
-high temperature
-high preassure
describe electrophillic addition of ethene and H-Br
-Electron pair in pi bond attracted to partially positive hydrogen atom.
-Pi bond breaks and hydrogen (of HBr) accepts a pair of electrons to form a bond with a carbon atom.
-At the same time, the H-Br bond breaks via heterolytic fission and the Br accepts a pair of electrons to form Br-
-The other carbon becomes positively charged and a carbocation intermediate is formed.
-Br- quickly donates a pair of electrons to the unstable carbocation to form bromoethane
describe curly arrows in the hydration of ethene
- curly arrow from pi bond to H+
-curly arrow from lone pair on partially negative O to C+
-curly arrow from O-H bond to O+
what is Markovnikov’s Rule
When a hydrogen halide reacts with an unsymmetrical alkene the halogen
of the hydrogen halide attaches itself to the carbon atom of the alkene with
the smaller number of hydrogen atoms and the greater number of carbons
what makes a carbocation more stable
-alkyl groups
-The more alkyl groups there are attached to a carbocation the more stable it is
-more alkyl groups releasing electrons towards positive charge
what is a primary carbocation
carbocation with one alkyl group
what is a secondary carbocation
carbocation with 2 alkyl groups
what is a tertiary carbocation
carbocation with 3 alkyl groups
what is the major product of electrophyllic substitution
for the molecule with the most stable
carbocation
why does having more alkyl groups make the carbocation more stable
-More alkyl groups = more of an inductive -effect (electron-donating ability).
-Alkyl groups have more electrons compared to hydrogen, which they can donate to the positive carbon atom via the covalent bond.
-Alkyl groups start to become more positive as a result and the carbon becomes less positive.
-There is more spread of charge across the carbocation, which makes it more stable.
-It takes less energy to make more stable carbocations and so these are formed more easily.
what are stereoisomers
-compounds that have the same structural
formula but different arrangements of their atoms in space
what must a molecule have to show E/Z isomerism
-a C=C double bond
-Each carbon in the double bond must have 2
different groups/atoms attached to it
why does e/z isomers exist
-because there is restricted rotation of atoms
about the double bond
-Double bond fixes atoms in position
