Basic Concepts and Hydrocarbons - alkenes Flashcards
Describe the bonding in alkenes
- c=c double covalent bond which has two sections involving the overlap electron clouds
- sigma bond is formed between the two carbon atoms using the direct overlap of electron clouds of the two atoms
- pie bond forms by electrons in adjacent p-orbitals overlapping above and below carbon atoms
- pie bonds are only made after sigma bonds
- the pie bond restricts rotation around the double bond making the molecule shape planar
Describe the shape of an alkene
120 degrees, and traigonal planar
- the covalent bond between the two carbon atoms are made from sigma and pie bonds this creates a high electron density by the same amount so form bond angles of 120
Basic things you need to know about alkenes
- hydrogen and carbon
- unsaturated
- CnH2n
- graduation in physical properties
- similar chemical properties
What does the C=C bond do
the C=C bond stops rotation this allows E/Z isomers to be formed
if two identical alkyl groups are attached Cis/Trans isomerism is formed
What is stereoisomerism
this is compounds that have the same structural formula but different arrangement of atoms in space
What happens in an addition reaction of alkenes
the pie bond will break first and react leaving the sigma bond between the two carbon atoms, the pie bond needs less energy to undergo homolytic fission therefore breaks more easily
- then the double bond is an area of high electron density which attracts electrophiles, for example Br2 which breaks the double bond, this makes the alkene saturated
Describe Hydrogenation
- it is an addition reaction
- Hydrogen is added across a C=C bond
- gaseous hydrogen and an alkene are mixed and passed over a nickel catalyst at 150 degrees in a reaction chamber
- saturates the alkene and products and alkane
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What is halogenation at test for? and explain
Test for saturation
- when a bromine or iodine is reacted with a saturated compound there is no reaction but with an unsaturated compound there is a reaction this changes the solution a different colour
Describe the basic way of doing halogenation
- a halogen is added across a C=C bond,
- alkene becomes saturated and a dihaloalkane is produced
Describe Halogenation
Step 1
- Bromine has non-polar molecules
- Bromine gets close to the electron rich C=C bond
- Dipole induced in Bromine
- slight positive is attracted to the high electron density of the C=C bond
Step 2
- Electrons from pie bond make a bond with a bromine ion
- this causes hetrolytic fission of the bromine molecule
- positive charge remains on a second carbon atom
- the species is very reactive and is a carbon cation
Step 3
- Two electrons from the bromide ion are shared with the carboncation
- makes a second bond and a stable product
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How does hydration work
- Addition reaction between a gaseous alkene and steam
- used to make alcohols
- condition involves high temperature (300 degrees) and pressures (65ATM) with a phosphoric acid catalyst (H3PO4)
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Describe the addition of hydrogen halides
Step 1
- large difference in electronegativity between hydrogen and bromine therefore the molecules is polar
- the slight positive hydrogen side of the HBr molecules is attracted to the high electron density of the C=C bodn
Step 2
- The electron in the pie bond makes a bond with hydrogen
- causes hetrolytic fission
- electron in the HBr molecule are given to the bromine atom - this makes a bromine ion
- a carbon cation is also made
Step 3
- the bromide ion and the carboncation bond to make a stable product
