ORGANIC CHEMISTRY - Synthesis

Question 1

what is bond fission

Answer 1

When an organic reaction takes place, bonds in the reactant molecules are broken and bonds in the product molecules are made.

The process of bond breaking is known as bond fission.

bond fission is the breaking of bonds

Question 2

the 2 types of bond fission

Answer 2

There are two types of bond fission, homolytic and heterolytic.

Question 3

homolytic fission

Answer 3

Homolytic fission:
♦ results in the formation of two neutral radicals
♦ occurs when each atom retains one electron from the σ covalent bond and the bond breaks evenly
♦ normally occurs when non-polar covalent bonds are broken
♦ two single-headed arrows starting at the middle of a covalent bond indicate homolytic bond fission is occurring

Reactions involving homolytic fission tend to result in the formation of very complex mixtures of products, making them unsuitable for organic synthesis.

(homo=same, 2 free radicals formed of the same charge)

electrons equally shared between the 2 atoms, 2 free radicals formed

Question 4

why reactions involving homolytic fission are unsuitable for organic synthesis

Answer 4

Reactions involving homolytic fission tend to** result in the formation of very complex mixtures of products**, making them unsuitable for organic synthesis.

Question 5

heterolytic fission

Answer 5

Heterolytic fission:
♦ results in the formation of two oppositely charged ions
♦ occurs when one atom retains both electrons from the σ covalent bond and the** bond breaks unevenly**
♦ normally occurs when polar covalent bonds are broken
♦ a double-headed arrow starting at the middle of a covalent bond indicates heterolytic bond fission is occurring

Reactions involving heterolytic fission tend to result in far fewer products than reactions involving homolytic fission, and so are better suited for organic synthesis.

(hetero=different, 2 ions of different charges created)

electrons shared unequally between atoms, 2 ions created

Question 6

why reactions involving heterolytic fission are suitable for organic synthesis

Answer 6

Reactions involving heterolytic fission tend to result in far fewer products than reactions involving homolytic fission, and so are better suited for organic synthesis.

Question 7

2 classifications of attacking groups in reactions involving heterolytic bond fission

Answer 7

In reactions involving heterolytic bond fission, attacking groups are classified as nucleophiles or electrophiles.

Question 8

what are nucleophiles

Answer 8

Nucleophiles are:
♦ negatively charged ions or neutral molecules that are electron rich, such as Cl- , Br- , OH- , CN− , NH3 and H2O
♦ attracted towards atoms bearing a partial (δ+) or full positive charge
♦ capable of donating an electron pair to form a new covalent bond

(electron-rich species thats seek out electron-deficient sites. may be uncharged molecules or negative ions, but must have at least one lone pair of electrons)

‘nucleus loving’

Question 9

what are electrophiles

Answer 9

Electrophiles are:
♦ positively charged ions or neutral molecules that are electron deficient, such as H+ , NO2+ and SO3
♦ attracted towards atoms bearing a partial (δ−) or full negative charge
♦ capable of accepting an electron pair to form a new covalent bond

(electron-deficient species that seek out electron-rich sites. usually positive ions or uncharged molecules with one atom that has a slightly positive charge (polar))

‘electron loving’

Question 10

what can partial charges on polar compounds act as

Answer 10

electrophilic or nucleophilic centres

Question 11

representation of carbon atoms in skeletal formula

Answer 11

In a skeletal structural formula, neither the carbon atoms, nor any hydrogens attached to the carbon atoms, are shown.

The presence of a carbon atom is implied by a** ‘kink’ in the carbon backbone**, and at the end of a line.

Question 12

what are haloalkanes

Answer 12

Haloalkanes (alkyl halides) are substituted alkanes in which one or more of the hydrogen atoms is replaced with a halogen atom.

Question 13

halogen prefixes for haloalkanes

Answer 13

Fluoro-
Chloro-
Bromo-
Iodo-

ending is longest alkane chain, halogen part comes before alkyl groups

Question 14

monohaloalkane facts

Answer 14

Monohaloalkanes:
♦ contain only one halogen atom
♦ can be classified as primary, secondary or tertiary according to the number of alkyl groups attached to the carbon atom containing the halogen atom
♦ take part in elimination reactions to form alkenes using a strong base, such as potassium or sodium hydroxide in ethanol
♦ take part in nucleophilic substitution reactions with:
— aqueous alkalis to form alcohols
— alcoholic alkoxides to form ethers
— ethanolic cyanide to form nitriles (chain length increased by one carbon atom) that can be hydrolysed to carboxylic acids

in Ethanol -> Elimination reaction

Question 15

nucleophilic substitution reaction mechanism types

Answer 15

A monohaloalkane can take part in nucleophilic substitution reactions by one of two different mechanisms – SN1 and SN2

The reaction mechanisms for SN1 and SN2 reactions can be represented using curly arrows

(nucleophilic substitution reactions involve an attacking nucleophile replacing a leaving group.)

Question 16

SN1

Answer 16

SN1 is a nucleophilic substitution reaction with one species in the rate determining step and occurs in a minimum of two steps via a trigonal planar carbocation intermediate.

• SLOW first step only involves ONE species (the haloalkane) reacting
• the ‘1’ also means its a FIRST ORDER REACTION (rate=k[…]^1)
• mechanism forms a true intermediate carbocation, as the cation is relatively stable.
• once carbocation formed, QUICKLY reacts with attacking nucleophile which is highly attracted to carbocation
• carbocation is planar, suggesting that substitution of the nucleophile may happen on either side, but some steric hinderance from departing halogen ion so nucleophile slightly favours opposite side.

SN1 = only 1 particle doing something in each step

Question 17

SN2

Answer 17

SN2 is a nucleophilic substitution reaction with two species in the rate determining step and occurs in a single step via a single five-centred, trigonal bipyramidal transition state.

• SN2 more likely to occur with a primary haloalkane
• the ‘2’ also means it is a SECOND ORDER REACTION (rate=k[…][…])
• the nucleophile approaches from the side away from the halogen (steric hinderance)
• a 5-centred transition state is formed but it is a one step reaction to the product

basically both the nucleophile and halogen are moving in the same step, so transition state made where carbon has 5 bonds (overall 1- ion, so use [ ]^-). use dotted lines in transition state to show the forming and breaking bonds (generally on opposite sides to carbon atom)

transition state very shortlived so not a ‘true’ step (more like a mini stage) so only really one step in mechanism.

SN2 = 2 particles involved in the one step

Question 18

explanations for which nucleophilic substitution mechanism preferred for a haloalkane

Answer 18

Steric hindrance and the inductive stabilisation of the carbocation intermediate can be used to explain which mechanism will be preferred for a given haloalkane.

primary/secondary haloalkanes tend to be **SN2
tertiary tends to be SN1
if compound can form a relatively stable positive ion (cation) then SN1 more favoured
more unstable cations will favour SN2 **mechanism
the more heavily substituted the cations are, the more stable they will be (so tertiary carbocations most stable)

methyl group electron rich, so the more methyl groups the more electron density around positive carbon centre, so C+ less attractive to nucleophiles. therefore carbocation more stable.

steric hinderance = physical blocking

Question 19

what is an alcohol

Answer 19

Alcohols are** substituted alkanes **in which one or more of the hydrogen atoms is replaced with a hydroxyl functional group, –OH group

Question 20

monohaloalkane reactions

Answer 20

elimination reactions to form alkenes using a strong base, such as potassium or sodium hydroxide in ethanol

nucleophilic substitution reactions with:
— aqueous alkalis to form alcohols
— alcoholic alkoxides to form ethers
— ethanolic cyanide to form nitriles (chain length increased by one carbon atom) that can be hydrolysed to carboxylic acids
— ammonia to form amines

in Ethanol -> Elimination reaction

Question 21

alcohol vs water solvent

Answer 21

Alcohol solvents tend to favour **ELIMINATION **(less polar)

**Water solvents **tend to favour SUBSTITUTION (more polar)

what type of reaction (elimination vs substitution) depends on the polarity of the solvent used.

Question 22

preparation of alcohol

Answer 22

Alcohols can be prepared from:
♦ haloalkanes by substitution
♦ alkenes by acid-catalysed hydration (addition)
♦ **aldehydes and ketones **by reduction using a reducing agent such as lithium aluminium hydride

(can’t create methanol by hydrating alkenes)

Question 23

Reactions of alcohols

Answer 23

Reactions of alcohols include:

♦ dehydration to form alkenes using aluminium oxide, concentrated sulfuric acid or concentrated phosphoric acid

♦ oxidation of primary alcohols to form aldehydes and then carboxylic acids and secondary alcohols to form ketones, using acidified permanganate, acidified dichromate or hot copper(II) oxide

♦ formation of **alcoholic alkoxides **by reaction with some reactive metals such as potassium or sodium, which can then be reacted with monohaloalkanes to form ethers

♦ formation of esters by reaction with carboxylic acids using concentrated sulfuric acid or** concentrated phosphoric acid **as a catalyst

♦ formation of esters by reaction with acid chlorides — this gives a faster reaction than reaction with carboxylic acids, and no catalyst is needed

acid chloride = a substituted carboxylic acid where -Cl replaces the -OH

Question 24

explaining alcohol physical properties

Answer 24

Hydroxyl groups make alcohols polar, which gives rise to hydrogen bonding.

Hydrogen bonding can be used to explain the properties of alcohols including boiling points, melting points, viscosity and solubility or miscibility in water.

Question 25

what are ethers

Answer 25

Ethers can be regarded as substituted alkanes in which a hydrogen atom is replaced with an alkoxy functional group, –OR, and have the general structure R’ – O – R’‘, where R’ and R’’ are alkyl groups.

Question 26

how to name ethers

Answer 26

Ethers are named as substituted alkanes.

The alkoxy group is named by adding the ending ‘oxy’ to the alkyl substituent, and this prefixes the name of the longest carbon chain.

eg methoxybutane

Question 27

preparation of ethers

Answer 27

Ethers can be prepared in a nucleophilic substitution reaction by reacting a monohaloalkane with an alkoxide.

Question 28

explaining physical properties of ethers

Answer 28

Due to the lack of hydrogen bonding between ether molecules, they have lower boiling points than the corresponding isomeric alcohols.

Methoxymethane and methoxyethane are soluble in water.
**Larger ethers are insoluble in water **due to their increased molecular size.

Ethers are** commonly used as solvents** since they are** relatively inert chemically** and will dissolve many organic compounds.

(ethers slightly polar as slightly kinked at -O- (lone pairs on O) so smaller ethers soluble in water)

only slightly polar so resist electophilic and nucleophilic attacks (so relatively chemically inert)

volatile and flammable

good solvent as dissolves both non polar and slightly polar molecules

solubility decreases as molecular size increases

Question 29

preparation of alkenes

Answer 29

Alkenes can be prepared by:
♦ dehydration of alcohols using aluminium oxide, concentrated sulfuric acid or concentrated phosphoric acid
♦ base-induced elimination of hydrogen halides from monohaloalkanes

when the group coming off is on a secondary carbon/not the end carbon, there will be more than one product (double bond can appear on either side of the carbon the leaving group left)

Question 30

reactions of alkenes

Answer 30

Alkenes take part in electrophilic addition reactions with:

♦ hydrogen to form alkanes in the presence of a catalyst
♦ halogens to form dihaloalkanes
♦ **hydrogen halides **to form monohaloalkanes
♦ water using an acid catalyst to form alcohols

electrophilic reactions as c=c bond electron rich and capable of donating an electron pair. electrophiles are attracted to c=c

Question 31

Markovnikov’s rule

Answer 31

Markovnikov’s rule states that when a hydrogen halide or water is added to an unsymmetrical alkene, the **hydrogen atom becomes attached to the carbon with the most hydrogen atoms attached to it already. **

Markovnikov’s rule can be used to predict major and minor products formed during the reaction of a hydrogen halide or water with alkenes.

rule can be explained by stability of carbocations, more stable carbocation will be favoured (so more likely that H joins to the carbon on the end, so resulting carbocation is secondary instead of primary.)

**major product produced from the more stable carbocation intermediate.
**

major product = the product that is most likely to be made.

Question 32

preparation of carboxylic acids

Answer 32

Carboxylic acids can be prepared by:
♦ oxidising primary alcohols using acidified permanganate, acidified dichromate and hot copper(II) oxide
♦ oxidising aldehydes using acidified permanganate, acidified dichromate, Fehling’s solution and Tollens’ reagent
♦ hydrolysing nitriles, esters or amides

Question 33

reactions of carboxylic acids

Answer 33

Reactions of carboxylic acids include:
♦ formation of salts by reactions with metals or bases
♦** condensation** reactions with alcohols to form esters in the presence of concentrated sulfuric or concentrated phosphoric acid
♦** reaction with amines** to form alkylammonium salts that form amides when heated
♦** reduction** with** lithium aluminium hydride** to form primary alcohols

Question 34

what are amines

Answer 34

Amines are organic derivatives of ammonia in which one or more hydrogen atoms of ammonia has been replaced by an alkyl group.

Question 35

classification of amines

primary, secondary, tertiary

Answer 35

Amines can be classified as primary, secondary or tertiary according to the number of alkyl groups attached to the nitrogen atom.

primary - one H replaced by an alkyl
secondary - two H replaced by alkyls
tertiary - three H replaced by alkyls

Question 36

reaction of amines

Answer 36

Amines react with acids to form salts.

(amides formed if the created salts are heated and lose water)

these are neutralisation reactions (but dont produce water, just getting rid of the acidity in the acid by taking the H+)

Question 37

amine bp trends

primary v secondary v tertiary

Answer 37

Primary and secondary amines, but not tertiary amines, display hydrogen bonding.

As a result, primary and secondary amines have higher boiling points than isomeric tertiary amines.

(tertiary amines doesnt hydrogen bond with itself, primary and secondary still have at least one N-H bond so have hydrogen bonding between their molecules)

Question 38

amine solubility trends

primary v secondary v tertiary

Answer 38

Primary, secondary and tertiary amine molecules can hydrogen-bond with water molecules, thus explaining the appreciable solubility of the shorter chain length amines in water.

amines have lone pairs on the N, so even tertiary amines can form hydrogen bonds with water.

the longer the nonpolar chains, the less soluble the molecule in water.

Question 39

basic properties of amines

(basic as in a base)

Answer 39

Amines like ammonia are** weak bases** and dissociate to a slight extent in aqueous solution.

The nitrogen atom has a lone pair of electrons which can accept a proton from water, producing hydroxide ions.

(OH left over after amine takes a proton from H2O)

amines are basic and can accept H+ via dative bonds to create ammonium parts (?)

Question 40

what is benzene

Answer 40

Benzene (C6H6) is the simplest member of the class of aromatic hydrocarbons.

Question 41

structure of benzene and its properties

Answer 41

The benzene ring has a distinctive structural formula. (hexagon with solid circle inside)

The stability of the benzene ring is due to the** delocalisation of electrons in the conjugated system. **

The presence of delocalised electrons explains why the benzene ring does not take part in addition reactions.

Question 42

explanation of bonding in benzene

Answer 42

Bonding in benzene can be described in terms of sp2 hybridisation, sigma and pi bonds, and electron delocalisation.

degenerate sp2 hybridised orbitals created from combining 2 p orbitals and 1 s orbital
3 sp2 hybridised orbitals per carbon
form 2 C-C σ bonds and a C-H σ bond
this leaves the remaining p orbital in carbon free
the p orbitals overlap which creates the delocalised electron ring below and above the carbon ring.

Question 43

what is a phenyl group

Answer 43

A benzene ring in which** one hydrogen atom has been substituted by another group** is known as the phenyl group. The phenyl group has the formula** –C6H5**.

phenYL (C6H5) is NOT phenOL (C6H5OH)

Question 44

reactions of benzene

Answer 44

Benzene rings can take part in** electrophilic substitution** reactions.

Reactions at benzene rings include:
♦ halogenation by reaction of a halogen using **aluminium chloride or iron(III) chloride for chlorination and aluminium bromide or iron(III) bromide for bromination
♦ alkylation by reaction of a haloalkane using aluminium chloride
♦ nitration using concentrated sulfuric acid AND concentrated nitric acid**
♦ sulfonation using concentrated sulfuric acid

attracts electrophiles since benzene has lots of electrons in the centre

halogenation - the catalyst polarises the halogen molecule creating an electrophilic centre

nitration - mixture of conc. nitric acid and conc. sulfuric acid known as a nitrating mixture, mix needed to create NO2+ ion (the electrophile)

HNO3 + 2H2SO4 –> H3O^+ + 2HSO4^- + NO2^+

sulfonation - works when reactant heated under reflux for several hours