alkenes Flashcards
(18 cards)
C=C double bond - general info.
On each C atom in the C=C bond there are 4 electrons orbitals, each containing one electron.
Three of the C orbitals create σ covalent bonds which have a trigonal planar arrangement.
The fourth C orbital (a p orbital) creates a π covalent bond which is perpendicular to the plane of the three σ covalent bonds.
σ covalent bonds.
description - made by the overlap of two orbitals. creates a molecular orbital that is directly between the two C nuclei.
rotation - there is free rotation around σ covalent bonds as this can happen without breaking the bond.
π covalent bonds.
description - made by the overlap of two p orbitals. creates a molecular orbital made up of two parts - one above and one below a line between the two C nuclei.
rotation - there is no free rotation around π covalent bonds as the bond would need to break.
What is the definition of stereoisomerism?
Molecules with the same molecular and structural formulas but a different arrangement of the atoms in space.
E-Z stereoisomerism in alkenes.
There is no rotation around the C=C bond as this would require the π bond to break.
If an alkene has two different groups on each C in the C=C then it has E and Z stereoisomerism.
Naming E-Z stereoisomers.
The atom numbers of the atoms bonded to the C atoms in the C=C are compared.
E = highest priority on opposite sides of the double bond.
Z = highest priority on the same side of the double bond.
*sides actually mean top/bottom
Electrophilc addition - general info.
Alkenes are susceptible to attack by electrophiles (lone pair acceptors).
This is because the C=C double bond is very electron rich due to the electron cloud of the π bond.
In an addition reaction, the C=C double bond opens up and an atom or group of atoms joins onto each C of the C=C double bond.
carbocations intermediates.
During the reaction, a carbocation is formed as an intermediate species in the mechanism.
Cations are positive ions (anions are negative ions).
Positive ions with the positive charge on the C atom are called carbocations.
Carbocations are very unstable as the C atom only has 6 electrons around it in the outer shell.
In terms of stability, tertiary carbocations are the most stable.
When alkenes undergo electrophilic addition reactions and different carbocations can be formed, the main product formed will be from the more stable carbocation intermediate. It is the structure and so stability of the carbocation intermediates that matters not the structure/stability of the product.
Answer to what is the major product?
Blahblahblah is the major product as it is produced via the most stable carbocation (tertiary vs secondary etc.)
Electrophilic addition 1 - outline.
reagent - Br2
conditions - aqueous
what happens - C=C opens up and a Br atom is added onto each C atom.
note - the Br-Br bond is not polar and so the Br2 molecule is not polar. however, as the Br2 molecule approaches the C=C, electrons are pushed away in the Br-Br making a temporary dipole. The delta positive end of the Br-Br bond is then attracted further to th C=C.
makes - halogenoalkanes
Electrophilic addition 2 - outline.
reagent - HBr
what happens - C=C opens up and a Br atom is added onto one of the C atoms with H on the other C atoms.
makes - halogenoalkanes
Electrophilic addition 3 - outline.
reagent - H2SO4
conditions - concentrated H2SO4, cold (typically room temp.)
what happens - C=C opens up and an H atom is added onto one of the C atoms with O-SO2-OH on the other C atom
note - the compound formed is an alkyl hydrogensulfate. their main use is to make alcohols. these are made by diluting (with water) and warming the reaction mixture.
makes - alkyl hydrogensulfates
Electrophilic addition 4 - outline.
reagent - H2O with strong acid (e.g. conc. H2SO4, H3PO4)
what happens - C=C opens up and an H atom is added onto one of the C atoms with OH on the other C atom
note - this reaction is reversible and the alcohol formed can react to reform the alkene.
makes - alcohols
Addition polymers.
Polymers are long-chain molecules made from many small molecules (monomers) together.
Addition polymers are made from monomers containing a C=C bond.
monomers - compounds with C=C
formation - polymer only
biodegradability - non-biodegradable
How do you name addition polymers?
poly(monomer)
Plasticisers.
Plasticisers are added to some polymers to make them more flexible (less brittle).
For example, plasticisers are added to PVC used to make the sleeves of electrical cables which need to be flexible.
Non-biodegradable.
Some polymers break down naturally over time (or when treated with bacteria/acids/alkalis).
However, addition polymers do not because their backbone chain is a chain of all C atoms and none of these C-C bonds are polar.
For many uses, it is helpful that they do not biodegrade (e.g. sleeves of electrical cables, pipes, artificial blood vessels), but it does mean that they would not decay if buried in landfill after use. These polymers should be re-used or recycled.
Development of polymers.
We have used natural polymers and some artificial polymers for many years.
Scientists first began to understand the structure polymers in the 1920s.
In the 1920s and 1930s many new polymers were developed (e.g. polythene, PVC, polystyrene).
New polymers with specific and unique properties continue to be developed.
However, we have become more conscious about the over-use of polymers and their disposal. Many polymers are used unnecessarily (e.g. disposable water bottles) but polymers are essential for many uses (e.g. sleeves foe electrical cables, pipes, artificial blood vessels).
