OCR A LEVELS - organic chemistry

how many bonds can a carbon form

4

define hydrocarbon

compound consisting of carbon and hydrogen

define saturated hydrocarbons

single carbon–carbon bonds only

define unsaturated hydrocarbons

the presence of multiple carbon–carbon bonds, including C=C, C C / and aromatic rings

define homologous series

a series of organic compounds having the ssame functional group but with each successive members differing by CH₂

define functional group

a specific group of atoms that are within organic molecules and are responsible for characteristic properties and reactivities of those molecules

define aliphatic

a compound containing carbon and hydrogen joined together in straight chains, branched chains or non-aromatic rings

define alicyclic

an aliphatic compound arranged in non-aromatic rings with or without side chains

define aromatic

a compound containing a benzene ring

what are the 3 homologous series of aliphatic hydrocarbons

• alkanes

• alkenes

• alkynes

name first 10 alkanes

1. methane = CH₄

2. ethane = C₂H₆

3. propane = C₃H₈

4. butane = C₄H₁₀

5. pentane = C₅H₁₂

6. hexane = C₆H₁₄

7. heptane = C₇H₁₆

8. octane = C₈H₁₈

9. nonane = C₉H₂₀

10. decane = C₁₀H₂₂

name first 10 alkyl groups (side chains)

1. methyl = CH₃

2. ethyl = C₂H₅

3. propyl = C₃H₇

4. butyl = C₄H₉

5. pentyl = C₅H₁₁

6. hexyl = C₆H₁₃

7. heptyl = C₇H₁₅

8. octyl = C₈H₁₇

9. nonyl = C₉H₁₉

10. decyl = C₁₀H₂₁

• R can be used to represent alkyl group

• general formula: C_nH_2n+1

how to name alkanes

1. identify longest chain of carbon atoms

2. identify any side chains attached to the parent chain

3. add numbers before any side chains to show position of the alkyl group

general formula for alkanes

C_nH_{2n+2 —→} contains a single C-C bond

naming alicyclic alkanes

1. identify continuous chain of carbon atoms

2. add the prefix cyclo infront of alkane

naming alkenes

1. identify the longest continuous chain of carbon atoms aka stem

2. identify where the double bond is

3. combine the stem with ene and the position of the double bond of the compound

general formula for alkenes

C_nH_{2n —→} contains a double bond

alcohol

functional group: -OH

prefix: hydrox-

suffix: ol

general formula: C_nH_2n+1OH

haloalkanes

functional group: -Cl -Br -I

prefix: chloro bromo iodo

aldehyde

functional group: -CHO c double bonds to O

suffix: -al

always on position 1

ketone

functional group: -C(CO)C- c double bonds to o

suffix: -one

general formula: C_nH_2nO

carboxylic acid

functional group: -COOH c double bonds to o

suffix: -oic acid

general formula:C_nH_2nO₂

naming alcohol

1. identify functional group and suffix

2. identify longest chain of carbon atoms

3. identify which carbon atom the functiona groups on

4. combine the suffix and stem and the position the functional group is on togather.

define general formula

the simplest algebraic formula of a member of a homologous series

e.g. for an alkane: C_nH_2n+2

define structural formula

the minimal detail that shows the arrangement of atoms in a molecule

e.g. for butane: CH₃CH₂CH₂CH₃ or CH₃(CH₂) ₂CH₃

• carboxyl group: COOH

• ester group: COO

define displayed formula

the relative positioning of atoms and the bonds between them

e.g. for ethanol:

define skeletal formula

the simplified organic formula, shown by removing hydrogen atoms from alkyl chains, leaving just a carbon skeleton and associated functional groups

e.g. for butan-2-ol:

displayed and skeletal for cyclohexane

for hexene it has one c=c bond and another line inside of hexagon

bezene ring

C₆H₆

define structural isomers

compounds with the same molecular formula but different structural formulae

isomers with the same molecular formula but different structural isomers

aldehydes and ketones = same molecular formula but different functional group

e.g. C₃H₆O

• aldehyde: propanal

• ketone: propanone

define homolytic fission

in terms of each bonding atom receiving one electron from the bonded pair, forming two radicals

• each atom has a single unpaired electron

• so a radical is formed

e.g H₃C-CH₃ —> H3C^. + ^.CH₃

ethane

define heterlytic fission

in terms of one bonding atom receiving both electrons from the bonded pair

• the atom that takes both electrons becomes negative ion

• atom that doesnt take electrons is positive ions

e.g H₃C-Cl —> H₃C+Cl^-

chloromethane

uses a curly arrow

bond fission occurs in

a covalent bond in alkanes

define radical

a species with an unpaired electron

‘dots’ to represent species that are radicals in mechanisms

Radical mechanisms will be represented by a sequence of equations.

Dots, •, are required in all instances where there is a single unpaired electron (e.g. Cl• and CH3•). Dots are not required for species that are diradicals (e.g. O)

define curly arrow

movement of an electron pair, showing either heterolytic fission or formation of a covalent bond

Curly arrows should start from a bond, a lone pair of electrons or a negative charge.

half headed arrows

is used for homolytic fission and represents the movement of a single unpaired electron mechanism involving radicals.

3 types of structural isomer

1. chain isomers

2. positional isomers

3. functional group isomers

define chain isomer

shown as skeletal formula

these isomers have similar chemical properties but their physical properties e.g. boiling point will be different as the shape of the molecule is different

define positional isomer

skeletal and functional group remains the same but the functional group is attached to a different carbon atoms

different physical and chemical properties

define functional group isomers

same atoms arranged in different functional group

so different physical and chemical properties

shape and bond angle of all alkanes

they are all tetrahedral shape around each carbon

these bonded pairs all repel equally

bond angle is 109.5

alkanes are held by what type of bonds

• alkanes consists of covalent bonds

• between the molecules there are induced dipole dipole interaction aka London Forces

the longer the carbon chain (straight chain) in alkane means

• stronger the induced dipole dipole interaction

• this is because theres more surface contact and more electrons to interact with

• more energy required to overcome to overcome the induced dipole dipole interaction - high BP

branched carbon chain alkane means

• theres a lower BP than straight chained isomer

• they cant pack closley togather

• they have a smaller molecular surface area

• this means induced dipole dipole interactions are reduced

complete combustion reaction: ALKANES

• formula: alkane + oxygen —> carbon dioxide + water

• combustion happens between gases, so liquid alkanes must be vapourised first. smaller alkanes turns into gases EASILY (more volatile) so burns easily

• larger alkanes releases ALOT of energy because they have more bonds to react too

• due to so much energy being released alkanes are excellent FUELS.

incomplete combustion reaction: ALKANES

formula: alkane + oxygen —> carbon monoxide + water

there is only a limited amount of oxygen in this reaction

carbon monoxide is poisonous

carbon monoxide binds better with haemoglobin in the bloodstream than oxygen, this means less oxygen can be carried around the body —- OXYGEN DEPRIVATION.

reaction of alkanes with halogens

under the condition of UV light, the initial energy is given to start the reaction

e.g. chlorine + methane —> chloromethane

_{overall reaction equation:} Cl₂ + CH₄ —> CH₃Cl + HCl

hydrogen atom is substituated by chlorine or bromine = free radical substituation reaction.

the 3 steps in the reaction mechanism for bromination/chlorination of alkane

1. Initation

2. Propagation

3. Termination

Initation of chlorination of methane

1. Initation - free radicals are produced

under UV light enough energy is provided to break the Cl-Cl OR Br-Br bond = photodissociation

the bond splits equally and each atom gets to keep one electron - homolytic fission

the atom becomes a highly reactive free radical because of its unpaired electrons

e.g. Cl₂ —> 2Cl^.

Propagation of chlorination of methane

2. free radicals are used and created in a chain

• Cl^. attacks the methane molecule — Cl^. +CH₄ —> .CH₃ + HCl

• the new methyl free radical ^.CH₃ can attack another Cl₂ molecule —- ^.CH₃ + Cl₂ —> CH₃Cl + Cl^.

• the new Cl^. can attack another CH₄ molecule until all the Cl₂ or CH₄ are all wiped out

Termination of chlorination of methane

3. Termination - free radicals are mopped up.

if two free radicals join togather they form a stable molecule

heaps of possible termination reactions can occur

Cl^. + ^.CH₃ —> CH₃Cl

^.CH₃ + ^.CH₃ —> C₂H₆

^.Cl + ^.Cl —> 2Cl

limitation of radical substituation

• Further substitution could occur if another chloride radical collides with the product of propagation.

• If the carbon chain is longer, a mixture of products forms as substitution occurs at different positions along the chain

alkenes are unsaturated because

they can make more bond with extra atoms in addition reaction.

what is a sigma bond

• it is formed when 2 s orbitals overlap

• these overlap in a straight line - giving the highest possible electron density between 2 nuclei - single covalent bond

• high electron density means strong electrostatic attraction between the nuclei and shared pair of electrons.

• means they have high bond enthalpy - strongest type of covalent bond

what is double bonds made up of

sigma bonds and pi bonds which makes alkenes more reactive due to the double bond

the C=C bond has 4 electrons so high electron density with pi bonds above and below meaning they are likely to get attacked by electrophile.

because C=C is very reactive handy starting point to making organic compounds e.g. petrochemical.

what is pi bond

• sideways overlap of two adjacent p orbitals

• got 2 parts to it one above and below the molecular axis

• these p orbitals are dumb bell shaped

• weaker bonds than sigma bonds as the electron density is spread out above and below the nuclei

• means the electrostatic attraction between the nuclei and shared pair of electrons are weaker

• so they have relatively low bond enthalpy.

shape of an alkene

• they are trigonal planar shape 120 degrees

• the 3 regions of electron density around each carbon atom means that they will repel each other as far apart as possible

• the C=C and the atoms bonded to these carbons all lie in the same planar

• in the imagine that is planar but if one of the H is replaced with CH₃ then it is non planar

why cant C=C atoms rotate

this is because the pi bonds locks the carbon atoms in position and prevents them from rotating around in the double bond

define stereoisomers

compounds with the same structural formula but with a different arrangement in space

define E/Z isomerism

• an example of stereoisomerism, in terms of restricted rotation about a double bond and the requirement for two different groups to be attached to each carbon atom of the C=C group

• E = enemies Z = same side

define cis/trans isomerism

• a special case of E/Z isomerism in which two of the substituent groups attached to each carbon atom of the C=C group are the same

• E = trans

• Z = cis

• only consistently correct when there is an H on each carbon atom of the C=C bond

using Cahn-Ingold Prelog priority rule to identify E and Z stereoisomers

in this system atoms attached to each carbon atom in a double bond are given priorities based on their atomic numbers

in the groups of higher priority are on the same side of the double bond = Z isomer

if the groups are placed diagonal across the double bond = E isomer

skeletal formula for but-2-ene

other way around for skeletal

Z = same side

E = enemies