Synthesis Flashcards
What is the term given to breaking bonds
Fission
What are the 2 types of fission
Homolytic
Heterolytic
Describe homolytic fission
Each electron in the sigma bond goes to a different atom
Two neutral species are formed
Where does homolytic fission occur
When non-polar covalent bonds are broken
What is the first stage of homolytic fission and how is it triggered
Initiation
UV
What do single-headed curly arrows symbolise
The movement of a single electron
Why are reactions involving homlytic fission unsuitable for producing organic compounds
Compounds will be produced in low yields and are difficult to isolate
Where does heterolytic fission occur
When polar covalent bonds are broken
Describe heterolytic fission
The more electronegative atom takes both of the electrons to make a positive and a negative ion
Why are heterolytic reactions more suitable for gaining products
Far fewer products are formed
What is a nucleophile
An electron donor
Negatively charged ion or a neutral molecule that is electron rich
What is an electrophile
An electron acceptor
Positively charged ion or neutral molecules that is deficient in electrons
What are aromatic hydrocarbons
Subset of hydrocarbons of which the simplest of these is benzene.
What is controversial surrounding the reactivity of benzene
It resists addition reactions - rapid decolourisation does not take place when added to bromine.
What type of bonding orbitals surround the carbon atoms in arenes (aromatic hydrocarbons)
sp*2 hybrid orbitals
- three of the half filled sp*2 orbitals form sigma bonds with a hydrogen and the two neighbouring carbons
- an electron is left occupying a p orbital on each carbon atom - these p orbitals overlap to form a pi molecuar orbital.
What is the total amount of orbitals found in benzene
12 sigma bonds
1 pi bond
Describe the 6 electrons that make up benzenes pi bond
Delocalised
What accounts for benzenes stability and why doesn’t it undergo addition reactions
Its delocalised electrons
Addition reactions would disrupt the electron delocalisation and would reduce its stability
Which reactions can occur on benzene without any disruption to its delocalised electrons
Substitution
Which aspect of benzene makes it susceptible to electrophiles
The delocalised electrons in the pi molecular orbital
List some electrophilic substitution reactions
Alkylation (CH3+)
Chlorination (Cl+)
Nitration (NO2+)
Sulfonation (HOSO2+)
What is the name of the benzene group in an alkane
Phenyl group.
What does aromatic mean
A system that contains a ring of atoms stabilised by delocalised pi electrons
Define aliphatic
molecules with straight or branched chains
Which benzene reactions are involved in dye production
Nitration
Alkylation
What can haloalkanes be regarded as
An alkane in which one or more hydrogens has been replaced by a halogen
What determines if a haloalkane is primary, secondary or tertiary
The number of alkyl groups attached to the carbon atom bearing the halogen atom
What is unique about the carbon atom of a monohaloalkane
It is polar since the halogen has a greater electronegativity
This makes the monohaloalkane acts as an electrophile
Why are monohaloalkanes susceptible to attack by nucleophiles
The halogen has a greater electronegativity which gives the carbon a slightly positive charge making it an electrophile
How do nucleophiles react with monohaloalkanes
The nucleophile donates an electron pair to the carbon atom of the c - x bond (x = the halogen) which forms a covalent bond.
The halogen atom is thrown out as a halide ion.
List some nucleophilic substitutions of monohaloalkanes
Monohaloalkanes react with:
Aqueous alkalis to form alcohols
Alcoholic alkoxides to form ethers
Ethanolic cyanide to form nitriles
How are alkoxides formed
When an alkali metal is added to an alcohol
How do you name nitriles
Prefix includes the amount of carbons of alkane and cyanide parts. eg. ethanenitrile = methane + cyanide
How is a nitrile converted into a carboxylic acid
Acid hydrolysis (with water and positive hydrogen ions
Which reaction does a monohaloalkane undergo if heated in reflux with ethanolic potassium (or sodium) hydroxide
Elimination
Describe an elimination reaction of a monohaloalkane
The halogen and a hydrogen on the adjacent carbon is removed from the alkane so it turns into an alkene - it is eliminated and not replaced
What is an reaction of a monohaloalkane when heated with potassium/sodium hydroxide called
Base-induced elimination reaction
What is unique about monohaloalkanes with more than 3 carbons that undergo elimination reactions
There is 2 possible products
What are the two different mechanisms by which a monohaloalkane will react under
Sn1
Sn2
What is the rate equation of a monohaloalkane undergoing Sn1 mechanism
Rate = k[(CH3)3CBr]
What is the meaning of Sn1
The reaction involves substitution by a nucleophile and that it follows first order kinetics (only one species is involved in the rate-determining step)
Describe the first step of the Sn1 process
Involves the slow-rate determining step and shows the C-Br bond undergo heterolytic fission to form a carbocation intermediate
Describe the second step of the Sn1 process
The fast step which involves the nucleophilic attack on the carbocation by the hydroxide ion
What is the rate equation of a monohaloalkane undergoing Sn2 mechanism
Rate = k[CH3CH2Br][OH-]
What is the meaning of Sn2
The reaction involves substitution by a nucleophile and that it follows second order kinetics (there are two species involved in the rate determining step)
What is the middle state of the Sn2 mechanism called
Transition state
What arrangement does the species in the transition state of an Sn2 mechanism adopt
trigonal bipyramidal
What types of carbocation can undergo the Sn1 mechanism
Primary, secondary, tertiary
What effect does alkyl groups have on the carbocation
A positive inducting effect
Describe the inductive effect
The alkyl groups are electron donating and can push electrons onto the positively charged carbon atom which stabilises the carbocation.
Which type of carbocation is the most likely to react with a nucleophile via an Sn1 mechanism and why
Tertiary because they are the most stable
What effect is associated with the size of the alkyl groups on a carbocation
Steric hindrance
Why are tertiary carbocations less likely to react with a nucleophile via an Sn2 pathway
The nucleophile is sterically hindered as the three bulky Sn2 groups limits the access to the carbocation
Which type of carbocation is most likely to react with a nucleophile via an Sn2 mechanism and why
Primary - there is only one alkyl group attached to the carbocation so access to the positively charged carbon will be much easier
What is the pathway a primary haloalkane is most likely to react via
Sn2
Why do alcohols have higher boiling points than most other molecules with similar amounts of electrons and similar shape
The polar OH- group forms hydrogen bonds with other molecules so require more energy to separate them
How does carbon chain length of an alcohol affect its solubility
The longer the carbon chain, the less soluble the alcohol (more non-polar and its more difficult for hydrogen bonds so they have a weaker effect)
List the ways of how alcohols can be made
By heating monohaloalkanes under reflux with aqueous potassium/sodium hydroxide (nucleophilic substitution)
Alkenes in the presence of water with a sulfuric acid catalyst (acid catalysed addition or hydration)
Aldehydes/ketones by reaction with lithium aluminium hydride dissolved in an ether
List the possible reactions of an alcohol
Alcohols react with:
Reactive metals such as sodium/potassium to form alkoxides (displacement/redox)
Aluminium oxide/concentrated sulfuric acid/concentrated phosphoric acid to form alkenes (dehydration/elimination)
Carboxylic acids/acid chlorides to form esters using a concentrated sulfuric acid catalyst [(condensation/esterification)(acid chloride is way faster and doesn’t require a catalyst)
What is the general structure of an ether
R-O-R’ where R and R’ are alkyl groups
What is the functional group of an ether
The alkoxy group
How do you name ethers
The shorter chain has ‘yl’ placed at the end of it eg. ‘methoxy’
The other alkyl group is placed after it
The number of the carbon on the larger alkyl group that the alkoxy group is attached to is placed in front of the name
What is the boiling point of ethers compared to alcohols and why
Lower - no hydrogen bonding between other ether molecules (the oxygen atom is not directly bonded to a hydrogen atom)
What explains the reason why ethers low in mass are soluble in water
They are able to form hydrogen bonds
What does it mean when large ethers are insoluble in water
They can be useful for extracting organic compounds from aqueous reaction mixtures
How flammable are ethers
Highly
Why do ethers make good solvents
Relatively inert chemically and most organic compounds dissolve them
Easily removed by distillation since they are volatile
How can you prepare an ether
Heating a haloalkane under reflux with an alkoxide (nucleophilic substitution)
Describe how alkenes can be prepared
Dehydration of alcohols - the vapour can be passed over hot aluminium oxide/alcohol can be treated with concentrated sulfuric acid/concentrated phosphoric acid
Base-induced elimination of hydrogen halides from monohaloalkanes - heating monohaloalkanes under reflux with ethanolic potassium/sodium hydroxide
Describe the process of dehydration
The OH- group is removed as well as a H on the adjacent carbon
What is another name for dehydration
Elimination
Why is concentrated phosphoric acid preferred over concentrated sulfuric acid when dehydrating alcohols
Sulfuric acid leads to more side reactions and may lead to charring
List the possible addition reaction paths of alkenes
React with:
Hydrogen to form alkanes (hydrogenation)(catalysed by nickel/platinum)
Halogens to to form dihaloalkanes (halogenation)
Hydrogen halides to form monohaloalkanes (hydrohalogenation)
Water to form alcohols (hydration)(catalysed by acids)
What is Markovnikov’s rule
The major product is the one in which the H atom ends up attached to the carbon atom of the double bond that has a greater number of hydrogens bonded to it.
Describe step 1 of halogenation
Bromine approaches the double bond and becomes polarised, electrons move to the bromine away from the double bond.
The positive halogen attacks the double bond and a cyclic intermediate is formed along with a negative halogen ion.
The halogen is acting as an electrophile and the double bonded molecule is the nucleophile
Describe step 2 of halogenation
The negative halogen ion attacks the cyclic intermediate ion on the opposite side.
The negative halogen ion is acting as the nucleophile and the cyclic intermediate ion is the electrophile
Describe step 1 of hydrohalogenation
The H-Br molecule is already polarised.
The positive H attacks the double bond forming an intermediate carbocation.
The H-Br molecule breaks heterolytically creating a negative Br ion.
Describe step 2 of hydrohalogenation
Involves the nucleophilic attack of the Br ion on the carbocation from either side.
If the molecule is unsymmetrical, the molecule with more alkyl groups attached to the carbocation will be more stable because of the positive inductive effect.
Describe step 1 of acid-catalysed hydration
A positive hydrogen ion bonds with a double bond which makes the alkenes carbon a carbocation
Describe step 2 of acid-catalysed hydration
The carbocation undergoes nucleophilic attack by a water molecule which makes a protonated alcohol
what is a protonated alcohol
An alcohol with an extra hydrogen on the OH group (a proton is a positive hydrogen)
Describe step 3 of acid-catalysed hydration
The protonated alcohol is a strong acid and readily loses a proton (hydrogen ion)
How can you prepare a carboxylic acid
Carboxylic acids can be prepared by:
Oxidising primary alcohols or aldehydes (heating under reflux with acidified potassium dichromate solution)
Hydrolysing nitriles (heated under reflux with an aqueous acid)
Hydrolysing esters (heated under reflux with an aqueous acid/alkali as a catalyst)
Hydrolysing amides (heating under reflux with an aqueous acid/alkali)
What reacts with a carboxylic acid to make salts
Metals
Carbonates
Alkalis
What do carboxylic acids react with (where no salt end product is formed)
Amines to form alkylammonium salts which form amides upon heating
Lithium aluminium hydride (LiAlH4) in ether to form primary alcohols (reduction)
What are amines
Organic derivatives of ammonia in which one or more hydrogen atoms in ammonia has been replaced by alkyl groups
How do you name amines
Usually put ‘amine’ after eg. methylamine 🙃
If the amide group is attached to a carbon in the middle of the chain: example = 2 aminopropane
Use for instance ‘ethyl
How do you name secondary/tertiary amines
Use prefixes ‘di’ and ‘tri’ respectively
Why do primary and secondary amines have hydrogen bonding
The N-H bond is polar
Why do tertiary amines not have hydrogen bonding with other tertiary amines
There is not a hydrogen directly connected to the nitrogen
Why do primary and secondary amines have higher boiling points than isomeric tertiary amines
They contain hydrogen bonds
Why are amines with low mass soluble in water
They can form hydrogen bonds with waters OH groups
How is an amine a weak base
The lone pair of electrons on the nitrogen accepts a proton from the water generating alkyammonium ions and hydroxide ions. The OH makes the solution alkali.
What do amines react with
Mineral acids to form salts
Carboxylic acids to form salts
What are the reagents and electrophiles in alkylation
R - CH3Cl/AlCl3
E - CH3+
What are the reagents and electrophiles in chlorination
R - Cl2/FeCl3
E - Cl+
What are the reagents and electrophiles in sulfonation
R - conc. H2SO4
E - HOSO2+
What are the reagents and electrophiles in nitration
R - conc. HNO3/H2SO4
E - NO2+
What is the name of new organic molecules which chemists are trying to make
Target molecules
List organic reactions
Substitution Addition Elimination Condensation Hydrolysis Oxidation Reduction
What is a functional group
The reactive part of a molecule which is responsible for the molecules reactions
How can you increase the chain length of a hydrocarbon
Adding a cyano group making it nitrile
