Organic chemistry - alkenes Flashcards
substituent
atom or group of atoms which could be replaced by a hydrogen atom
test for alkenes
bromine water
(addition reaction)
drawing alkene
when drawing molecules/bonds a carbon is always bonded to central C not a hydrogen e.g C=C and CH3 or H3C depending on its position
suggest why a molecule may not be named using the cis-trans naming system
-it is a larger organic compound
-has more than one atom in common
which statement about ethene is correct
it burns in excess oxygen to produce CO2 and water
identify the feature of the double bond in the E and Z isomers that causes them to be stereoisomers
restriction rotation around the pi bond
Describe the two methods for forming a haloalkane
-addition of hydrogen halides (reacts with alkenes at room temperature)
-addition of halogens added to alkenes at room temp
addition
hydrogenation –> C=C + H2
Halogenation –> C=C + Br2
state what reagents are required to make propan-2-ol
-prop-2-ene
-steam
-phosphoric acid catalyst
alkene + water (steam) and phosphoric acid catalyst =
alcohol
explain why Br-Br is attracted to the C=C of the alkene
-C=C is an electron rich region
-Br-Br becomes polarised (not polarised before due to same electronegativity) when near the alkene double bond
-this means that the partially positive Br atom becomes attracted to the C=C
suggest why an electrophile would react with ethene not ethane
ethane has no electron rich area and no pi bond
arrangement of bonds around C=C
The arrangement of bonds around the
>C=C< is planar and has the bond angle 120
C=C double covalent bond
consists of one sigma (σ)
bond and one pi (π) bond.
stereoisomers
Stereoisomers have the same structural formulae
but have a different spatial arrangement of atoms.
Alkenes can exhibit a type of isomerism
called E-Z stereoisomerism
what are alkenes
-alkene –> homologous series of hydrocarbons that have an unsaturated bond between 2 carbon atoms (CnH2n)
-alkenes can be straight chained or cyclical
single vs double bonds
-A single bond is able to rotate freely which means that the above molecules are the same and are not isomers of each other
-A double bond is not able to rotate freely. This means that the above molecules are considered isomers of each other, locked in positions
sigma bond
A single covalent bond is known as a sigma bond which are formed by the overlapping of atomic orbitals
Pi bond (single)
Pi bonds –> formed by the lateral overlapping of two lobes –> additional type of bond found in molecules with double and triple bonds
geometric isomers
-There is restricted rotation around the C=C double bond which results in the existence of geometric isomers
-When theres a double bond (or pi bond) if we wanted to rotate the atoms around it we would need to break the pi bond. However, this is not possible at room temperature because it would require energy to break the bond so free rotation is not possible around the C=C in alkenes. This forms a type fo stereoisomer
when do E/Z isomers form
-molecules have restricted rotation about a bond
-have different groups attached to the carbon atoms at the end of each bond
Z isomer
Z isomer = functional groups on the same side (cis)
E isomer
E isomer = function groups on opposite sides (trans)
trans vs cis
If an alkyl group or atom other than hydrogen is attached to each carbon then the isomers can be named cis (same side) or trans (opposite side)
Cis molecules = top or bottom are the same –> usually polar molecules with the same side groups on the same side of the double C bond
Trans isomer = same side groups placed on opposite sides of the double bond
complex organic compounds
-In more complex organic compounds where multiple hydrogens have been substituted by different group isomers, it cannot be define as cis-trans notation. Instead we use E/Z isomer notation
E/Z isomer
-isomers are defined as E or Z depending on what priority is given to the groups attached to the carbon atoms in the C=C
-E and Z isomers arise due to the double bond
-When there is more than 2 different groups around the alkene double bond we cannot have E/Z isomerism, instead it is only a geometric isomer
assigning priority
-We need to determine which groups we give priority to when there are four different groups on the carbon atoms in the double bond
-We give the highest priority to the atom with the highest atomic number
addition reactions
-Addition reactions occur with alkenes in which one molecule combined with another molecule to form a larger molecule with no other products
what do alkenes undergo addition reactions with
-hydrogen
-halogens
-hydrogen halides
-steam (H2O)
addition of hydrogen
-hydrogen can be added to the C=C double bond in alkenes to form an alkane e.g ethene + hydrogen –> ethane
-The conditions required for this is room temperature in the presence of a platinum or palladium nickel catalyst
-hydrogenation
catalytic hydrogenation
-used in the manufacture of margarine
-oily unsaturated liquids are turned into soft saturated fatty solids like margarine using a nickel catalyst
addition of halogens
-Halogens can be added to alkenes but at room temperature to form a haloalkane
-Flourine reacting with small alkenes is relativey explosive but with iodine the reaction is slow
testing for alkenes
-orange –> colourless (bromine water)
-undergoes an addition reaction to create a haloalkane which turns colourless
-unsaturated bond decolorises
-when solution becomes colourless Br2 undergoes an addition reaction
addition of hydrogen halides
-react readily with alkenes at room temperature to form a haloalkene
-hydrogen halides react with alkenes in the gas phase
addition of steam
-Alkenes react with steam in the presence of an acid catalyst (phosphoric acid catalyst or sulfuric acid) to produce alcohols
-This is a reversible reaction
Oxidation by potassium manganate (KMnO4):
-Potassium manganate can oxidise alkenes in the presence of sulfuric acid
Alkenes can be oxidized to produce cis-diols using a different type of reagent that adds atoms across the double bond via a cyclic intermediate
-during the reaction MnO4- ions are reduced to Mn2+
-colour change = purple to colourless
But-1-ene can also react with acidified potassium manganate solution. This is both an oxidation and addition reaction
Butan-1,2-ol –> OH group on 1st and 2nd carbon
reactivity of C=C
-the electron density is above and below the plane of the molecule
-The double bond of the alkene is an area of high electron density and is therefore an area of high negative charge
-The exposed electrons of the pi bond may be attacked by electron deficient species (those with partial positive or actual positive charges)
electrophiles
-electrophiles are molecules or ions that attack negative ions or parts of molecules which are rich in electrons with negative centres
-electrophiles attack alkene bonds or ions (positive charge or partial positive)
-Electrophiles are also described as electron acceptors bc they accept electrons from the molecule or ion they attack
explain why ethane undergoes substitution and not addition reactions
ethane has no double carbon bonds that can be shared
explain why more of isomer E than isomer F is formed in this reactioj
-different stabilities in carbocations
what happens during electrophilic addition
-When an electrophile such as H-Br, H-H or Br-Br is added to an alkene we call it electrophillic addition (always occurs with double bonds or electron dense areas)
-An alkene reacts with an electrophile to produce 1 product only
-The Pi bond breaks and the two carbons form new bonds
