Alkenes Flashcards
what is the general formula of alkenes?
CnH2n
What is a sigma bond
a bond formed by the lengthways overlap of orbitals directly between 2 bonding atoms
what is a pi bond?
the sideways overlap of a spare unbonded p orbital present on each carbon atom in the double bond
what forms a pi bond
overlap produces a cloud of electron density above and below the molecule
What shape and angle is the double C=C bond
Planar
120°
because no free rotation around the double bond
what is structural isomerism
molecules with the same molecular formula but different structural formula
what is stereoisomerism
molecules with the same structural formula but different arrangement of atoms in space
what is a Z isomer
when the functional groups are on the same side of the carbon eg both on top or both on the bottom
Zame side
How does geometric isomerism occur
restricted rotation around the double bond
2 different groups are bonded to each C atom in the C=C
When does E isomerism occurs
when the functional groups are on different sides
eg one top left and one bottom right
(opposite)
What are cis isomers
when the same functional groups are on the same side
eg 2 CH3 on the top
what are trans isomers
the same functional groups on opposite sides
eg CH3 top left and CH3 bottom right
What are the 4 addition reactions of alkenes?
Hydrogenation
Halogenation
Addition of an acid
Hydration
What is hydrogenation and what reagents are required for it
Addition of hydrogen
Reagents : H2 (gas) or Nickel
Double bond breaks and Hydrogen added
What is the importance of hydrogenation
used in the manufacture of margarine
What is halogenation and what are the reagents for it
Addition of a halogen
Regents : Br2 or Cl2
Double bond breaks and Br - Br or Cl-Cl added
What is the importance of halogenation
used in the test tube lab test for the presence of an alkene
What are the reagents required for the addition of an acid
HBr or HCl
what is the importance of adding an acid
used in organic synthesis when making a haloalkane
What is hydration and what are the reagents required
Addition of water
Reagents : steam/temp > 100
and Catalyst : H2SO4 or H3PO4
What is the importance of hydration
used to make industrial ethanol for fuel and solvent purposes
What is an electrophile
an electron pair acceptor
What is the electron dense region
The C=C bond is a region of high electron density and attracts electrophoresis
Also attracts delta positive
What happens when the C=C bond breaks in electrophilic addition
the alkenes undergo addition reactions and form a single bond
This allows each carbon atom to form a bond with other atoms forming saturated (single bonds) C-C products
Step 1 for electronic addition
curly arrow from double bond to delta positive atom
Step 2 for electrolytic addition
curly arrow from bond between X-Y to delta negative (Y)
step 3
double bond breaks C
carbocation formed : + on the carbon bonded to more carbons
:Y- becomes a nucleophile
step 4
curly arrows from :Y- electron pair to carbocation
Y added to the carbocation
how do u test for alkenes (unsaturated compounds) (double bonds)
when alkenes are added to bromine water a yellow colour is recolonised
this shows the presence of a C=C double bond
how does Br2 become polar for electrophilic addition
electron dense area of the carbon double bind induces a dipole ok the bromine molecule
it repels the electrons in the Br-Br so it becomes polar
How many products are formed from an unsymmetrical alkene
2
major and minor product
When is the major product formed
Major product is formed via the most stable cation (tertiary)
Most stable cation = carbon bonded to the most other carbons
when is the minor product formed
from the least stable carbocation
what is addition polymerisation
joining monomers (alkenes) to form polymers (poly(alkenes) which are unreactive
unreactive because the C-C bind is single and non polar
how do y draw a repeating unit
double bond broken
extended bonds sticking out the side
brackets
n outside the brackets
n = number of repeating units
what can waste polymers be used for
recycled/reused but not all
2 alternatives to recycling plastics
What is combustion for energy production
polymers burned to release heat energy for generating electricity
Adv : reduced the need for combusting fossil fuels
Dis : combustion of chlorine containing pointed produce HCL gas which is toxic and corrosive
Releases CO2 - global warming
Releases toxic CO
what is PVC
poly chloro ethene
hard, rigid because permanent dipole dipole forced between chains due to delta position and negative Cl
How is PVC made more flexible
Plasticisers are added to the PVC to reduce the effectiveness of these attractions
what are the problems with plastics
non biodegradable due to strong non polar C-C bonds
How can HCL gas be prevented from entering the atmosphere
reacting it with bases eg CaCO3 or CaI to neutralise it
how to minimise the dangers of toxic CO
entire there’s plenty of oxygen to prevent incomplete combustion
what is organic feedstock
raw materials for making chemicals
advantage : reduces the need for cracking crude oil
