basic organic concepts and alkenes

Homologous series definition

-a series of compounds with the same functional group and therefore similar chemical properties

-each successive member differs by CH2

-as you move along, there’s a gradual change in physical properties due to increasing molecular size and mass

What’s a functional group?

-a group of atoms in an organic molecule that’s responsible for the characteristic reactions of that molecule

-e.g. double bond in alkenes

What is a pi bond?

-a bond formed by the sideways overlap of 2 p-orbitals

-pi bond forms above and below plane of molecule

-in a C=C double bond, it locks both C atoms in position and prevents them rotating around the double bond (restricted rotation)

What’s a pi orbital?

-pi bond

-they lie above and below rest of molecule

-p-orbitals don’t overlap as effectively as s-orbitals so pi bonds are weaker, and alkenes are more reactive than alkanes

What’s in a double alkene bond?

-one of the shared pairs lies in a molecular orbital on the line between 2 nuclei (sigma bond), formed by overlap of 2 s-orbitals

-the other pair is found above and below plane of molecule, it’s a pi-bond, formed by the sideways overlap of 2 p-orbitals

Why are alkanes unreactive?

-C-H bonds is non-polar and short, so nuclei are close to shared pair of electrons and the bond is strong

-for a sigma bond, the overlap is along the line directly between 2 nuclei, higher e- density, stronger than pi bond

-alkane molecule is a tetrahedral shape, bond angle 109.5

What happens to physical properties of alkanes as number of carbons increases?

-boiling point increases, molecule gets bigger, more electrons per molecule, induced dipole forces get stronger

-viscosity increases

-flammability decreases

-volatility decreases (ability to evaporate)

What are structural isomers?

-molecules with the same molecular formula but different structural formula

What is an aliphatic molecule

-an organic molecule containing H and C that doesn’t contain a benzene ring (non-aromatic)

e.g. alkanes, cycloalkanes

What’s an aromatic compound

-any organic compound that contains benzene ring

What does alicyclic mean?

-any carbon ring that isn’t the benzene ring

-cyclic is any ring

general formula: CnH2n (same as alkenes)

alcohol general formula

CnH2n+1 OH

empirical formula definition

simplest whole number ratio of atoms of each element in a compound

structural formula definition

the arrangement of atoms in a molecule without showing all the bonds

-e.g.CH3CH2CH3

Skeletal formula

-show the bonds of the carbon skeleton only

-carbon and hydrogen atoms aren’t shown but functional groups are

Displayed formula definition

show the arrangement of atoms showing all the bonds and atoms in a molecule

ketones

aldehydes

cycloalkanes

esters

-alkyl…anoate

carboxylic acid

CnH2nO2

Heterolytic fission

-a way which a covalent bond can break

-pair of electrons moves to one of the atoms and you end up with 2 ions

e.g. C=O > C-O:-

electrophile

-a species that accepts a pair of electrons

rules for naming alkanes

-find longest continuous carbon chain

-identify and name groups attached to chain

-number the chain consecutively, starting at the end nearest a substituent group

-designate the location of each substituent group by a number and name

-assemble name, listing groups alphabetically

-prefixes (di, tri) aren’t considered when alphabetising

E/Z isomerism checklist

-an alkene which needs to have 2 different groups attached to each C of the C=C

-and if one of the groups on both carbons is a H, we call the stereoisomers cis/trans rather than E

-(special case of E/Z isomerism)

stereoisomers

-molecules with the same molecular and structural formula but have different spatial arrangements of atoms

-e.g. E/Z stereoisomers

How to draw a reaction mechanism

1- show bond polarities using partial charges

2-show transfer of pair of electrons using curly arrows

3-show reaction intermediates including charges as necessary

E/Z isomerism (Cahn, Ingold, prelog)

-due to double bond, rotation round double bond is restricted

-they’re compounds with different spatial arrangements of groups attached to carbons

-there must be 2 different groups on each carbon of the C=C

-CIP rules are that whichever element directly attached to carbon has a higher atomic number is priority group

electrophilic addition mechanism for ethene and water to make an alcohol

-necessary conditions are that water has to be vapour, over 100 degrees C(steam), and a phosphoric/sulphuric acid catalyst

-ethanol

electrophilic addition reaction for ethene and H2 to make alkane

-necessary conditions: nickel catalyst and H2

-curly arrow from double bond to delta + H

-curly arrow from H-H bond to delta - H

-intermediate is carbocation

-curly arrow from H- (hydride) ion to carbocation

-product is ethane (alkane)

tertiary carbocations (markownikoff’s rule)

-ion with positive carbon atom with 3 alkyl groups

-more likely to form than secondary, which are more likely to be formed than primary

-as positive charge is stabilised by alkyl groups

-so tertiary carbocation intermediates are the most stable

-so will be formed in greater proportion

-(H add onto alkene carbon with the most H already attached)

When can there be 2 possible products in electrophilic addition?

-if both electrophile and alkene are asymmetric

-alkene doesn’t have mirror plane through C=C

dehydration of cyclohexanol

-heated with concentrated phosphoric /sulphuric acid

-the liquid cyclohexene distills off and collected and purified

markownikoff’s rule

-major product from addition of a hydrogen halide to an unsymmetrical alkene is the one where hydrogen adds to carbon with the most hydrogens already attached

What’s the shape and bond angle around a C=C

-trigonal planar

-120 degrees

How can waste polymers be processes to reduce their environmental impact?

-cracked to give monomers then processes into new plastics (recycled)

-sorted and remoulded (recycled)

-combustion to produce energy, burnt as fuel

why do straight chain molecules have higher boiling points than branched molecules

-can pack more tightly

-more contact surface area

-stronger induced dipole forces

Use of methanol

-manufacture esters

-solvent

alternatives for polymers

-biodegradable polymers

-photodegradable (chemically broken down by sunlight)

-used as organic feedstock

bromine test for alkenes

orange to colourless

polypropene

double bond is broken

molecular formula

shows all atoms of each element in a molecule

e.g. C6H12O