Topic 6: Organic Chemistry I

Question 1

what is meant by ‘homologous series’?

Answer 1

A family of compounds w/ the same functional group. They differ by CH2 from the next member, with similar chemical and a trend in physical properties

Question 2

what is meant by ‘functional group’?

Answer 2

an atom/group of atoms in a molecule responsible for su chemical reactions, replacing H in an organic compound

Question 3

What are isomers?

Answer 3

Compounds w the same molecular formula but different structures.

Question 4

What is the difference between position, functional group and structural isomers?

Answer 4

Position isomers have the functional group of the molecule in a different position of the carbon chain, eg propan-1-ol and propan-2-ol. They have similar chem but diff physical properties.

Functional group isomers have the same molecular formula but the molecules have diff functional groups, chemical and physical properties, eg hexene and cyclohexane.

Structural isomers have the same molecular formula but a different structural arrangement of atoms. They can be straight or branched chains, eg butane vs 2-methylpropane.

Question 5

What is stereoisomerism?

Answer 5

Stereoisomers have the same structural formula but have a different spatial arrangement of atoms and bonds.

E/Z is a type of stereoisomerism which occurs due to limited rotation around a C=C.

The limited rotation means that groups attached to the C=C can either be on the same side or on opp sides.

Showing the structure with atoms in a straight line doesn’t show stereoisomerism.

Question 6

How do you assign priority to E/Z isomers?

Answer 6

In molecules that display E/Z isomerism, the atom or group on each side of the double bond with the higher Mr is given the higher priority. These groups are used to determine if the molecule is the E or Z isomer. If the two atoms attached to the C=C are the same, you need to find the first point of difference after that and then assign priority.

Question 7

How is E/Z isomerism different to cis trans isomerism?

Answer 7

Cis and trans can ONLY be used when there are hydrogen atoms to compare the other groups to.

Question 8

What happens to the boiling temp of the alkanes if the chain is straight or branched?

Answer 8

longer chain= more e and more points of contact between molecules so more London forces. This increases bpts

Branching lowers bpt due to less points of contact between molecules.

Question 9

Describe and explain the types of fission.

Answer 9

Bond fission is the different ways in which the chemical bond between 2 atoms can be broken.

Heterolytic fission is when a cv bond breaks and one atom takes both electrons. 2 ions, one + and one - are formed.

Homolytic fission is when a cv bond breaks and each atom takes an electron. Two radicals formed.

A radical is an atom or group of atoms with an unpaired electron.

Question 10

Show the mechanism of how to form chloromethane (halogenation substitution)

Answer 10

Initiation: Cl2—UV LIGHT)–> 2Cl°

Propagation 1: Cl° + CH4 —–> °CH3 + HCl

Propagation 2: °CH3 + Cl2 —–> °Cl + CH3Cl

Termination 1: °CH3 +CH3 —–> C2H6

Termination 2: Cl° + Cl° ——-> Cl2

Termination 3: °CH3 + Cl —–> CH3Cl

In the products, the total number of free radicals decreases. ° symbol means a free radical

Question 11

Why are chain reactions are not a good method for preparing halogenoalkanes?

Answer 11

If an alkane is more than 2 Cs in length, then any H atoms would be substituted, leading to a mix of different isomers. The mixture of products is hard to separate, so it is not a good method.

Some chloromethane molecules formed during the free radical substitution between CH4 and Cl will undergo further substitution to form dichloromethane.

Further substitution can occur till ALL H’s are substituted. These extra chloroalkanes are impurities that must be removed.

The amount of these molecules can be decreased by reducing the proportion of chlorine in the reaction mixture.

Question 12

Give disadvantages of fermentation of ethanol

Answer 12

In fermentation, hay an upper limit to the conc of ethanol in the solution.

The ethanol must be separated from a lot of water, which needs energy.

Making ethanol from bacteria and plant waste instead has a much higher upper limit for the conc of ethanol in the solution.

Question 13

What are the advantages and disadvantages of plastics/polymers made from alkenes?

Answer 13

They are chemically unreactive so useful for making pipes, packaging.

Easily moulded into attractive shapes.

Lightweight, can easily be coloured

Disadvantages: Low mpts.

When they burn they release toxic gases.

It has C-Hal bonds which bacteria cannot break so they’re non-biodegradable

Question 14

Describe Alkenes

Answer 14

Alkenes/cycloalkenes are unsaturated hydrocarbons with at least one C=C. Not found naturally in crude oil or natural gas.

Part of a homologous series with the general formula CnH2n. CycloalkAnes are saturated and follow this same general formula.

The C=C in alkenes makes them reactive. During su reactions, the C=C opens up to form single bonds w other atoms.

The bond angle around C=C is 120° due to the overlap of P orbitals. The shape around each carbon is trigonal planar

Question 15

What is special about the C=C?

Answer 15

The C=C is an area of high electron density making it susceptible to attack from electrophiles (species attracted to electron dense areas). It consists of a normal covalent sigma bond and a pi bond

Question 16

How do you test for alkenes?

Answer 16

Alkenes decolourise bromine water to change colour from orange/brown to colourless.

This is because the C=C bond can open up to accept bromine atoms, and thus become saturated.

Question 17

Describe and explain the boiling point of alkenes, and how they combust.

Answer 17

As chain length increases bpt increases due to more e and therefore more Ldn forces that need more energy to break.

Slightly lower bpt than alkanes bc of fewer electrons, and the chain is more kinked.

They burn with a smokey flame due to higher C:H ratio, so it’s not used as fuels

Question 18

Describe alkene solubility and density

Answer 18

Insoluble in water – but soluble in each other.

All liquid alkenes are less dense than water and will therefore be the upper organic layer in chemical reactions.

The density of the alkenes increases with increasing chain length.

Question 19

Why are alkenes more reactive than alkanes?

Answer 19

Alkenes have a pi bond.

Alkanes have a sigma bond which is stronger than a pi bond.

Therefore alkenes are more reactive

Question 20

What is electrophilic addition with hydrogen?

Answer 20

Alkenes can undergo electrophilic addition with hydrogen to form alkanes in a hydrogenation reaction.

Catalytic hydrogenation is used to manufacture margarine from unsaturated vegetable oils.

Hydrogenation strengthens the chain which increases bpt bc of more points of contact between molecules which is more Ldn forces.

Conditions for hydrogenation: room temp with a platinum catalyst or 150 degrees with a nickel catalyst

Question 21

What are electrophiles?

Answer 21

Electrophiles are electron acceptors and are attracted to areas of high electron density.

The most common are HBr, Br2, H2S04.

Electrophiles can be used in the presence of steam to form alcohols or with hydrogen to form alkanes from alkenes

Question 22

Draw the mechanisms for: electrophilic addition of hydrogen bromide to ethene electrophilic addition of hydrogen bromide to propene

Answer 22

In electrophilic addition of hydrogen bromide to propene, 2-bromopropane forms more often than 1-bromopropane.

This is bc the H joins to the carbon atoms bonded to the most hydrogens.

The Br atom joins to the C atom joined to the most carbons.

Question 23

Draw the mechanism for Ethene + Bromine

Answer 23

Question 24

Why do haloalkanes contain polar bonds?

Answer 24

The halogens are more electronegative than a carbon atom.

This means electron density is drawn towards the halogen, forming a slightly positive and slightly negative charge.

Question 25

What are primary, secondary and tertiary halogenoalkanes?

Answer 25

The C atom directly bonded to the halogen has a max of 1 other C atom directly bonded to it.

Secondary: The C atom directly bonded to the halogen has 2 other C atoms directly bonded to it.

Tertiary: The C atom directly bonded to the halogen has 3 other C atoms directly bonded to it. Eg 2chloromethylpropane

Question 26

Describe reactivity of the haloalkanes

Answer 26

Electronegativity of halogens decreases down the group, so a C-F bond is much more polar than a C-I bond. The C-F bond is also shorter, so its way stronger, so less reactive.

The greater the Mr of the halogen of the polar bond, the lower the bond enthalpy. The bond can be broken more easily, increasing reactivity.

So, halogens lower down in the group increase reactivity: iodo>bromo>chloro>flouro. Tertiary> secondary> primary. Tertiary is most reactive due to the stability of the carbocation intermediate following the heterolysis of the C-X bond.

Question 27

What affects the boiling points of the Haloalkanes?

Answer 27

As the length of the alkyl chain increases so does bpt.

Halogens further down the group attached to the hydrocarbon increases its bpt because of more electrons and ldn forces.

Question 28

Describe the hydrolysis of haloalkanes with silver nitrate.

Answer 28

Haloalkanes can be broken down in a reaction w aqueous silver nitrate and ethanol. This can test for the haloalkane.

The water in the solution acts as a nucleophile which breaks down the haloalkane, releasing halide ions into the solution. The halide ions then react with the silver ions to form silver ppts.

White, cream or yellow ppt should form and depending on the halide they might dissolve in dilute or conc ammonia.

The faster the ppt forms, the less stable the haloalkane is. The reaction is nucleophilic sub. Primary is SN2, with a transition state and [RCH2X] [OH-] in RDS. Tertiary is SN1, with a carbocation and only [RCH2X] in RDS.

Question 29

What are nucleophiles?

Answer 29

These species are positive liking.

They contain a lone e pair that is attracted to the slightly positive region of molecules.

Some of the most common nucleophiles are: CN:- :NH3 -:OH

Question 30

How do you make an alcohol from a haloalkane? Draw the mechanism for it, using bromoethane and aqueous potassium hydroxide as an example.

Answer 30

Heating a haloalkane with aq NaOH under reflux makes an alcohol. The attacking nucleophile is the OH- ion.

E.g. 1-chloropropane converted to propan-1-ol. Bromoethane is converted to ethanol.

The bond breaking in this mechanism is Heterolytic fission, and the reaction is nucleophillic sub (SN2)

Question 31

How do you make nitriles from haloalkanes?

Answer 31

Heating haloalkanes under reflux with potassium cyanide dissolved in ethanol makes nitriles.

The attacking nucleophile is the CN- ion. E.g. Bromoethane + KCN —> propanenitrile + KBr

Note the organic product contains 1 more C atom than the starting material, so this reaction is useful to extend the C chain.

Question 32

How do you make primary amines from haloalkanes?

Answer 32

Heating haloalkanes w ammonia solution in a sealed tube makes primary amines. The sealed tube is needed because ammonia gas otherwise escapes from the apparatus before it could react. The attacking nucleophile is the NH3 molecule.

The first organic product is a base which reacts with the hydrogen halide product to form a salt, not a primary amine. To produce a high yield of amine, ammonia is used in excess.

So, the final products are the primary amine and the ammonium halide.

Question 33

Draw the mechanism of the reaction between chloroethane and ammonia heated in a sealed tube.

Answer 33

This reaction forms a PRIMARY AMINE, which forms ethyl amine.

Question 34

Describe elimination reactions of haloalkanes. use 2-bromopropane + ethanolic KOH as an example

Answer 34

When a haloalkane is heated w ETHANOLIC Na/KOH, the OH- acts as a base and not as a nucleophile.

Here, OH- reacts w the hydrogen attached to the carbon next to the C-Br bond to form h20.

Eg: 2-bromopropane + ethanolic KOH–> prop-2-ene + H20 + KBr

H and Br are removed from the haloalkane but aren’t replaced by any other atoms, so the reaction is called elimination.

Question 35

What is the difference between primary, secondary and tertiary Alcohols?

Answer 35

P:The carbon atom directly bonded to the OH has a max of one other carbon atom directly bonded to it

S:The carbon atom directly bonded to the OH has 2 other carbon atoms directly bonded to it

T: The carbon atom directly bonded to the OH has 3 other carbon atoms directly bonded to it

Question 36

Describe combustion of alcohols.

Answer 36

Alcohols are used as biofuels. if combustion is complete, the products are carbon dioxide and water.

Question 37

How do you convert an alcohol into a haloalkane?

Answer 37

To convert an alcohol into a haloalkane you replace the OH group in an alcohol w a halogen atom. The reaction is called halogenation.

But, a different method is needed for each halogen. Hay 3 types of reactions: Chlorination, bromination and iodination.

Question 38

Describe chlorination of alcohols

Answer 38

Chlorination conditions: PCl5 at room temp

Hay also 2 inorganic products: phosphorus oxychloride and HCl: ethanol + PCl5—–> chloroethane + POCl3 + HCl

This can also test for an OH group, as the white misty HCl fumes can be seen and tested with damp litmus paper.

Chlorination of tertiary alcohols can also be done a second way: by mixing the alcohol with conc HCl at room temp. H2O is also formed as part of the products

Question 39

Describe bromination of alcohols

Answer 39

Bromination conditions: use a mix of potassium bromide and 50% conc H2SO4, and warm with the alcohol.

It’s better to write 2 equations rather than 1 as the inorganic reagents react together to form HBr:

2KBr + H2SO4—–> K2SO4 + 2HBr (inorganic reagents)

C2H5OH + HBr—–> C2H5Br + H2O (organic reaction)

Question 40

Describe iodination of alcohols, using ethanol as an example

Answer 40

Iodination conditions: use a mixture of red phosphorus and iodine. Heat with the alcohol under reflux.

As with bromination, write 2 equations as the inorganic reagents first react to form phosphorus iodide:

2P + 3I2 → 2PI3.

PI3 then goes on to attack the alcohol: 3C2H5OH + PI3 —> 3C2H5I + H3PO3

Question 41

What is dehydration reaction of alcohols?

Answer 41

Heating an alcohol with conc phosphoric acid removes the OH group and a H atom.

A C=C is formed in the carbon chain.

Water is the only inorganic product.

Question 42

Describe the dehydration reaction with butan-2-ol

Answer 42

Heating an alcohol with conc phosphoric acid removes the OH group and a H atom. The 2 possible products are but-1-ene and but-2-ene:

CH3CH(OH)CH2CH3 –> CH2=CHCH2CH3 + H20

CH3CH(OH)CH2CH3 –> CH3CH=CHCH3 + H20

But-2-ene exists as a pair of E/Z isomers. So, hay 3 products: but-1-ene and the 2 isomers of but-2-ene.

The formula for phosphoric acid doesn’t appear in the eqn; the water formed in the reaction mixes with the conc phosphoric acid to dilute the acid.

Question 43

Describe the reagents needed for the oxidation of alcohols

Answer 43

A mixture of potassium dichromate and dilute sulphuric acid. The oxidising agent is represented by [O].

Whenever this oxidising agent is used, hay a colour change from orange to green. This can be used as a test for OH groups

Other oxidising agents: Potassium permanganate, Air + Silver catalyst

The reaction with these oxidising agents does not affect tertiary alcohols, so NO colour change

Question 44

Generally, how does oxidation affect alcohols?

Answer 44

Unlike dehydration, oxidation only affects one carbon atom.

The atoms removed from an alcohol are the hydrogen of the OH group and a H atom from the carbon atom joined to the OH group as shown:

Question 45

Why can’t tertiary alcohols be oxidised?

Answer 45

Only the primary and secondary structures have a hydrogen atom on the C of the C-OH group, the tertiary structure does not.

Question 46

Describe oxidation of a primary alcohol with Acidified Potassium Dichromate (K2Cr2O7/H2SO4)

Answer 46

Primary alcohols become oxidised to form aldehydes (the =O is in the end of the C chain). Unlike ketones, aldehydes are more easily oxidised than alcohols. So when a primary alcohol is oxidised the aldehyde formed may be oxidised further (by gaining 02) to carboxylic acids.

This further oxidation occurs if the oxidising agent is in excess, and the ethanal is prevented from evaporating.

During this reaction, K2Cr2O7 is reduced to hydrated Cr3+ ions. A colour change from orange to green happens.

Question 47

Describe the oxidation of a secondary alcohol.

Answer 47

Add acidified potassium dichromate and heat under reflux. The secondary alcohol becomes oxidised to form ketones (the =O is in the centre of the C chain)

During this reaction, K2Cr2O7 is reduced to hydrated Cr3+ ions. A colour change from orange to green happens

Question 48

How do you test for purity?

Answer 48

Finding the bpt of a compound and compare w data book value. Purer substances are closer to the databook bpt. Low purity samples boil over a range of temperatures.

This test may not be conclusive bc of inaccuracies in measuring boiling temp so you may wrongly assume your compound is pure.

Also, some organic compounds coincidentally have the same bpts.

Question 49

What happens to an alkene if you add potassium premanganate solution?

Answer 49

They are oxidised at room temperature to diols.

Eg ethene + potassium manganate(VII) —> Ethan-1,2-diol

Question 50

How do you form methanol?

Answer 50

React CO with Hydrogen: 2H2(g) + CO(g) → CH3OH(g)

Cond: High temp, Zinc and Chromium oxide catalyst

The CO and H2 can be made from reacting Coal/Methane with steam

Question 51

How do you form ethanol?

Answer 51

React ethene w steam: (alkenes+ steam makes alcohols).

H2O(g) + C2H4(g) → C2H5OH(l)

Cond: 360 degrees, 60 atm pressure. H3PO4 catalyst

OR Fermentation of glucose: C6H12O6 → 2C2H5OH + 2CO2

Cond: 37 degrees C, yeast catalyst. 15% ethanol production