3.5 Synthesis Flashcards
Unit 3: Organic Chemistry
Types of reaction: Addition
Two reactant molecules join to form one product molecule
Types of reaction: Substitution
One or more atoms are substituted between molecules
Types of reaction: Elimination
One or more atoms are removed from a molecule, leaving at least two products
Types of reaction: Condensation
Two molecules join with the loss of a small molecule, usually water
Types of reaction: Hydrolysis
-Water is used to split a large molecule into two
-The H from the water attaches to one part and the OH attaches to the other part leaving two new smaller molecules
Types of reaction: Oxidation
Oxidation of an organic molecule increases the O:H ratio in the molecule (more oxygen to hydrogen)
Types of reaction: Reduction
Reduction of an organic molecule decreases the O:H ratio in the molecule (less oxygen to more hydrogen)
Types of reaction: Neutralization
Neutralization of an organic acid will always produce a salt and water as products, and carbon dioxide if the base is a carbonate
What is Bond Fission?
When bonds in the reactant molecules are broken, this requires energy
What is Homolytic Fission?
This is when a sigma covalent bond is broken evenly, each of the atoms involved retains one of the two electrons from the bond, this happens if the electrons in the bond are shared equally part of a non-polar bond - as a result two neutral radicals are formed (these are highly reactive)
What is a Free Radical?
An atom or molecule containing an unpaired electron. It has an equal number of protons and electrons, so has no overall charge
What are the 3 reactions with radicals?
-Initiation
-Propagation
-Termination
Initiation
The UV light supplies enough energy to break the bond in the bond, if the bond is not polar than homolytic bond fission occurs leaving two radicals
Propagation
These are the reactions which keep the chain reaction going. In each propagation step one radical enters the reaction and another is formed
Termination
These reactions remove free radicals from the system without replacing them with new ones bringing the chain reaction to an end. When two radicals combine a stable molecule is formed
Why are radicals not useful for synthesis?
A mixture of compounds can result, and the more complex the alkane, the more complex the mixture of products made, this makes these types of reaction unpredictable
What is Heterolytic Fission?
This occurs when a polar covalent bond is broken. It breaks unevenly, so both of the electrons from the sigma covalent bond are retained by the more electronegative atom. The more polar a bond, the more likely the fission will be hetrolytic. As a result two oppositely charged ions are produced
Why is heterolytic fission more useful in organic synthesis?
There are fewer products in reactions involving heterolytic bond fission
What does curly arrow notation show?
The movement of electrons during bond fission and bond formation
What would be seen in curly arrow notation for homolytic bond fission?
One electron foes to each atom forming radicals so 2 single head arrows leaving the bond between molecules going to each other
What would be seen in curly arrow notation for heterolytic bond fission?
Both electrons go to one atom forming two oppositely charged ions so one double headed arrow going to the negatively charged ion
What are Nucelophiles?
They are chemical species that carry a negative charge, or can be neutral but have an area of high electron density (non-bonding pairs of electrons/polar bonds). They can donate a pair of electrons to allow the formation of a new dative covalent bond
What are nucelophiles attracted to?
Atoms with a full or partial positive charge
Examples of nucelophiles:
Cl-, Br-, OH-, CN-, NH3 and H2O
What are Electrophiles?
Postive ions or neutral molecules that are deficient in electrons. They can accept an electron pair and from a dative bond
What are electrophiles attracted to?
Negative ions or atoms with a partial negative charge
Examples of electrophiles:
H+,NOx+ and SO3
What is a leaving group?
A fragment of a molecule that leaves with a pair of electrons in heterolytic bond fission. A good leaving group must be able to stabilize the additional electron density that results from bond fission
Examples of leaving groups:
-Halides (Cl-, Br- and I- but not F-)
-Neutral leaving groups - water, ammonia
-Few goof cationic leaving groups, but the hydronium ion is one
What occurs in the elimination reaction when forming an alkene from a haloalkane and what catalyst/condition is required?
The nucleophile attacks the H atom, donating a pair of electrons to form a new bond. The electrons from the C-R Bond (R= haloalkene) forming a bromide ion which is the leaving group. The electron pair from the C-H bond form a second bond with neighbouring carbon
-Strong base in ethanol is required
What are the 2 steps of a Sn1 reaction?
- The leaving group leaves with a pair of electrons leaving behind a triganal planer carbocation
- The nucleophile attacks the carbocation, attaching to the +ve carbon atom, creating the new substituted molecule
What conditions/catalyst is required for a Sn1 reaction?
Acqueous alkalo (KOH / NaOH)
How does the alkyl groups affect the stability of the carbocation?
As the alkyl groups increases the stability increases, this is because they have a tendency to push electrons towards a neighboring carbon atom, and this helps stabilize the positive charge on the carbon atom. This is called inductive stabilization
What is forms in the transition state (-ve) in a Sn2 reaction?
An intermediate triganal byprymidal
What occurs in an Sn2 reaction and what conditions/catalysts are required?
-The nucleophile attacks the carbon attached to the halogen donating a pair of electrons to create a bond
-Transition state formed
-Electron pair from C-Halogen bond moves to the halogen atom forming halide ion
-Halide ion leaves alcohol is the substituted product
Requires aqueous alkali
Why is the Sn2 mechanism difficult for a tertiary haloalkane?
If a tertiary haloalkane was to be substituted, the large space occupied by the alkyl groups would block access to the +ve carbon
-this is known as steric hinderance
What determines whether a secondary haloalkane is an Sn1 or Sn2 mechanism?
The nature of the solvent that the reaction is taking place in
What is an Ether?
A molecule made up of two alkul groups joined through an atom of oxygen
What is an Alkoxide?
-These are ions that are formed by the loss of a hydrogen ion from the hydroxide group of an alcohol
-These are formed by the reaction of an alkali metal with an alcohol
How does an Monohalo alkane form a ether and what condition/catalyst is required?
(substitution) - Sn2
-Nucleophilic alkoxide attacks haloalkane
-Transition state forms
-Polar C-monohalo alkane splits heterdyilcally
-Electron pair transfer to monohalo alkane to from ion - leaving group
-New bond forms from non-bonding pair of electrons of oxygen between the O and the C to form the ether
-Requires alcoholic alkoxide
How does a monohaloalkane form a nitrile and what conditions/catalyst is required?
Nucleophile (C triple bond N) attachs the haloalkane, this causes a new bond to form between the two C atoms. The polar C-halogen bond splits, electron pair forms a halide ion
Why is the formation of a nitrile from a monohaloalkne useful in organic synthesis?
-The nitrile product contains one more carbon atom in its chain than the orignal haloalkane
-This is therefore a useful method of increasing the carbon chain length of an organic compound
What is the simple way of preparing alkenes from alcohol?
The alcohol vapour is passed over heated aluminium oxide powder, and the ethanol is “cracked” to give ethene and water vapour
How can you prepare an alkene from an alcohol using either conc phosphoric acid or conc sulfuric acid?
-The hydroxyl is protonated by the acid catalyst
-Water molecule leaves
-Carbocation loses a hydrogen ion
-Pi bond forms to give C=C
What catalyst is required to form an alkane from an alkene through addition (hydrogenation)
Nickel (Ni) or Pallodium (Pd)
How does a alkene form a haloakane through hydrohalogenation and what catalyst/conditions are required?
As the hydrogen halide approchese the alkene a bond forms between the H and one of the carbon atoms, and the hydrogen halide breaks with the halide taking both electrons. The halide ion acts as a nucleophile and attacks the carbocation that has been formed
-Requires a halogen halide
How does a alkene form a dihaloalkene through halogeneration and what catalyst/conditions are required?
The bond between the halogen breaks the same time the C=C Pi bond breaks, this then causes a bond to form between the halogen and C-C forming a Halogen ion. This then attacks one of the carbon atoms and forms a dihaloalkene
-Halogen is required
What is Markovnikov’s rule state?
When a hydrogen halide or water is added to an unsymmetrical alkene, the hydrogen atom attches to the carbon with the most hydrogen atoms attached to it already
Why is Markonvnikov’s rule useful in organic chemistry?
It can be used to predict the major and minor products that are formed when an alkene undergoes an addition reaction with a hydrogen halide or water
How does an alkene form an alcohol through the addition of water (water is required)
The proton from the acid attacks one of the carbon atoms in the double bond resulting in the formation of a carbocation. The nucleophile (water) attacks the C+ to form a second intermediate. This second intermedite loses a proton to form ethanol
How can alcohols be prepared?
-Subsitution of haloalkenes
-Hydration of alkenes
-Reduction of aldehydes and ketones
What is a good reducing agent?
Lithium aluminium hydride
How does a nitrile produce an acid and ammonia
If it is heated under reflux with an aqueous acid catalyst (supplies H+ ions)
How can amides by hydrolysed?
-Heating under reflux with an aqueous acid or alkaline catalyst
-This produces an alkaline catalyst shifts the equilibrium to the right to favour the products
What is the general mechanism for an electrophilic subsitution reaction of benzene?
-The electrophilic attacks one of the carbon atoms in the ring, forming a positively charged intermediate ion that is stabilised by delocalisation around the ring
-The intermediate ion then loses H+ to regain aromatic character and restore the system
What happens in a halogenation reaction?
One of the hydrogen atoms is substituted with a hydrogen atom to form halobenzene
What happens in Alkylation?
One of the hydrogen atoms is substituted with an alkyl group in the presence of an aliumium chloride catayst. This reaction is carried out using FeCl3 for chlorinaion or FeBr3 or AlBr3 for bromination
What happens in Nitration?
One of the hydrogen atoms is subsituted with a nitro (NO2) group in the presence of conc sulfuric and conc nitric acid
What happens in Sulfonation?
One of the hydrogen atoms is substiuted with a sulfonyl group to from benzene sulfonic acid, this reaction proceeds when benzene is heated under reflux or several hours with conc sulfuric acid
What 4 addition reactions can Benzene under go?
-Halogenation
-Alkylation
-Nitration
-Sulfonation
