Hydrocarbons Flashcards
What is homolytic fission?
The breaking of a covalent bond with one of the bonded electrons going to each atom, forming two radicals.
What is heterolytic fission?
The breaking of a covalent bond with both of the bonded electrons going to one of the atoms, forming a cation and an anion
What is an addition reaction?
A reaction in which a reactant is added to an unsaturated molecule to make a saturated molecule
What is a free radical substitution reaction?
one or more hydrogen atoms is removed from a saturated hydrocarbon and replaced by another atom
What is an elimination reaction?
The removal of a small molecule from a saturated molecule to make an unsaturated molecule
What is an electrophile?
An electron pair acceptor
(An atom (or group of atoms) that is attracted to an electron-rich centre or atom, where it accepts a pair of electrons to form a new covalent bond)
What is a nucleophile?
AN ELECTRON PAIR DONOR
An atom (or group of atoms) that is attracted to an electron- deficient centre or atom, where it donates a pair of electrons to form a new covalent bond.
Describe the bonding in alkanes
Covalent bonds between C-H and C-C formed by the overlap of orbitals creating sigma bonds consisting of one electron from each bonding atom.
Carbon always forms 4 bonds with surrounding atoms while H only forms 1
Describe the shape of alkanes (bond angle)
Each carbon is surrounded by 4 electron pairs which repel each other to form bond angles of 109.5°. Long chain alkanes do not have fixed shapes, they could be zigzagged or they could form a U shape.
What is the effect of chain length on boiling point of alkanes
As the chain length increase, more London forces are present between molecules as there is more surface area contact. The more London forces require a larger amount of energy to overcome therefore the boiling point is higher on longer chained alkanes.
What is the effect of branching on boiling point of alkanes?
Alkanes with more branches have less surface area contact between each other meaning that there are less London forces present, therefore requiring less energy to overcome, meaning alkanes with more branching have lower melting points.
Why is UV light required when reacting halogens with alkanes?
When reacting a halogen with an Alkane to form a haloalkane, UV light must be present as it provides the molecules with sufficient activation energy for the reaction to take place.
Describe the initiation stage of bromination
The halogen (Br2) is broken down into radicals with free electrons (Br• + Br•)
Describe the propagation stage of bromination
Their are two stages to propagation:
Stage 1- Br• is used to form a •CH3 radical and a HBr molecule
Stage 2- a Br2 molecule and the •CH3 form CH3Br and Br•
The cycle then repeats, the Br• used in stage 1 is formed in stage 2
Limitations of radical substitution
- Further substitution (CH3Br, CH2Br2, CHBr3 or CBr4)- Substitution on different points on the chain (isomers can be formed)
What is a free radical?
A species with an unpaired electron
What is electrophilic addition?
An addition reaction in which the first step is attack by electrophile on a region of high electron density.
What is electrophilic substitution?
A type of substitution reaction in which an electrophile is attracted to an electron rich centre atom, where it accepts a pair of electrons to from a new covalent bond
Describe the nature of the double bond in alkenes
A sigma bond is formed between the C-C and then two p orbitals from each Carbon overlap above and below the plane of the sigma bond forming a pi bond.
Describe the shape of the double bond
Trigonal planar- there are three regions of electron density forming bond angles of 120°.
How does E/Z isomerism occur?
Rotation around the double bond is limited, meaning alkenes with the same molecular formulas can have different arrangements. The molecule must have C=C bonds and have different groups bonded to each C=C carbon.
Describe Cis-Trans Isomerism
Cis-trans isomers have the same criteria as E/Z but with one extra rule- one of the groups attached to the C=C carbons must be the same, like in But-2-ene.
Describe the Cahn-Ingold-Prelog nomenclature
Each group attached to the C=C carbons is given a priority based on their atomic number. If both high priority are above or below the C=C carbons it’s a Z isomer, whereas if a high priority and a low priority are above or below the C=C carbons it’s an E isomers
What are the conditions required for hydrogenation of an alkene, and what is formed?
Hydrogenation of alkenes requires a high temperature and a nickel catalyst, this forms the corresponding alkane.
What are the conditions and products of halogenation of alkenes?
Halogenation of alkenes can occur rapidly at room temperature and a haloalkane is formed (e.g. 2-bromoethane)
Describe the test for saturation in in an unknown solution
Halogenation of alkenes can be used to determine the presence of C=C bonds. When bromine water is added dropwise to a sample if the solution turns colourless from orange then alkenes were present, if the solution remains orange no C=C bonds are present.
Addition of hydrogen halides to alkenes
Alkenes react with gaseous hydrogen halides and HCl(aq) and HBr(aq) at room temperature
What are the conditions and products of hydration reactions of alkenes
Alkenes react with steam in the presence of a H3PO4(aq) catalyst and from the corresponding alcohol.
Describe the But-2-ene + HBr mechanism
HBr becomes dipolar with H as the + pole as Br is more electronegative. The electron rich pi bond attracts the polar H which causes heterolytic fission between H and Br and the double bond to break which forms a carbocation. The Br- ion forms a bond with the carbocation and 2-bromobutane is formed.
Describe the propene + bromine mechanism
When Bromine, a non polar molecule, approaches propene the pi interact with the Br-Br bonding electrons which forms an induced dipole. The pi bond is attracted the to induced positive Br which causes the double bond to break. Br bonds to one of the carbons which leaves a Br- ion and a carbocation, these then bond and 1,2 dibromopropene is formed.
Markownikoff’s rule
Markownikoffs rule predicts the most common product of addition when an unsymmetrical alkene forms two isomers. This is in the form of a major and a minor product. The minor product is when the halogen bonds to a primary carbocation while the major product bonds the halogen to a secondary carbocation.
Briefly describe the carbocation stability trend
Primary is less stable than secondary which is less stable than tertiary which is the most stable carbocation.
What is addition polymerisation?
Unsaturated alkene molecules (monomers) form long chains saturated chains (polymers)
Environmental concerns with polymers
Very unreactive so they do not biodegrade, manufactured from finite fissile fuels that are unsustainable and release greenhouses gases when they’re made. Dangerous to wildlife if irresponsibly disposed of.
