synthesis Flashcards
sn2
nucleophilic substitution
2nd order
2 species in RDS
Transition step intermediate
one step mechanism
primary/ secondary tendancy
sn1
nucleophilic substitution
1st order kinetics
one species in RDS
carbocation intermediate
2 step mechanmism
tertiary tendancy
why do tertiary halo alkanes react via the sn1 mechanism?
because tertiary carbocations are stable
define steric hindrance
when the bulky groups are blocking the space where the nucleophile needs to attack
what uses the sn1 mechanism
tertiary haloalkanes and haloalkanes with steric hindrance
state features of homolytic fission
Homolytic fission:
results in the formation of two neutral radicals
occurs when each atom retains one electron from the sigma covalent bond and the bond breaks evenly
normally occurs when non-polar covalent bonds are broken
what are the 2 types of bond fission
There are two types of bond fission, homolytic and heterolytic
what happens when an organic reaction takes place
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
what do the reactions involving homolytic fission tend to result in
Reactions involving homolytic fission tend to result in the formation of very complex mixtures of products, making them unsuitable for organic synthesis
state features of heterolytic features
Heterolytic fission:
results in the formation of two oppositely charged ions
occurs when one atom retains both electrons from the covalent bond and the bond breaks unevenly
normally occurs when polar covalent bonds are broken
what do reactions involving heterolytic fission result in
Reactions involving heterolytic fission tend to result in far fewer products than reactions involving homolytic fission, and so are better suited for organic synthesis
what are nucleophiles
♦ 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
what are electrophiles
♦ 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
how can synthetic routes be devised
Synthetic routes can be devised, with no more than three steps, from a given reactant to a
final product.
how can the possible reactions of a particular molecule be deduced
The possible reactions of a particular molecule can be deduced by looking at the structural
formula.
explain skeletal structural formula
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
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
what can monohaloalkanes react with
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
what are haloalkanes
Haloalkanes (alkyl halides) are substituted alkanes in which one or more of the hydrogen
atoms is replaced with a halogen atom.
what are the nucleophilic substitution reaction mechanisms that mono-haloalkanes can undergo?
A mono haloalkane can take part in nucleophilic substitution reactions by one of two different mechanisms.
sn1 and sn2
what is SN1
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.
what is SN2
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
how can SN1 and SN2 be represented
The reaction mechanisms for SN1 and SN2 reactions can be represented using curly
arrows. Steric hindrance and the inductive stabilisation of the carbocation intermediate can
be used to explain which mechanism will be preferred for a given haloalkane.
alcohols
Alcohols are substituted alkanes in which one or more of the hydrogen atoms is replaced with a hydroxyl functional group, –OH group.
how can alcohols be prepared
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
how can ethers be produced?
Ethers can be prepared in a nucleophilic substitution reaction by reacting a monohaloalkane with an alkoxide.
how are ethers named
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.
why do ethers have a lower boiling point
Due to the lack of hydrogen bonding between ether molecules, they have lower boiling points than the corresponding isomeric alcohols.
what do hydroxyl groups make alcohols
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.
how are alkenes prepared
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
Alkenes take part in electrophilic addition reactions with:
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
How can carboxylic acids can be prepare
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
Reactions of carboxylic acids include:
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
what are amines
Amines are organic derivatives of ammonia in which one or more hydrogen atoms of
ammonia has been replaced by an alkyl group.
what does Markovnikov’s rule state
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
what can Markovnikov’s rule be used to predict
Markovnikov’s rule can be used to predict major
and minor products formed during the reaction of a hydrogen halide or water with alkenes.
The reaction mechanisms for the addition of a hydrogen halide and the acid-catalyzed addition of water …
The reaction mechanisms for the addition of a hydrogen halide and the acid-catalyzed addition of water can be represented using curly arrows and showing the intermediate
carbocation. The inductive stabilisation of intermediate carbocations formed during these reactions can be used to explain the products formed.
how can amines be classified
Amines can be classified as primary, secondary or tertiary according to the number of alkyl
groups attached to the nitrogen atom.
Amines react with acids to form…
Amines react with acids to form salts.
which amines display hydrogen bonding
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.
Primary, secondary, and tertiary amine molecules can hydrogen bond with water molecules, thus explaining…
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.
how do amines produce hydroxide ions
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.
what does the benzene ring have
The benzene ring has a distinctive structural formula. The stability of the benzene ring is due
to the delocalization of electrons in the conjugated system
what does The presence of delocalised electrons explain in benzene
The presence of delocalised
electrons explains why the benzene ring does not take part in addition reactions.
what is benzene
Benzene (C6H6) is the simplest member of the class of aromatic hydrocarbons.
what can bonding in benzene can be described in terms of
Bonding in benzene can be described in terms of sp2 hybridisation, sigma and pi bonds, and electron delocalisation.
what is a phenyl groups
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
Benzene rings can take part in electrophilic substitution reactions. Reactions at benzene
rings include:
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
