3b- Synthesis Flashcards
What happens when an organic reaction takes place?
Bonds in the reactant molecules are broken and bonds in the product molecules are made
What is the process of bond breaking known as?
Bond fission
What are the two types of bond fission?
Homolytic and heterolytic
What are 3 key facts about about 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 3 key facts about about heterolytic fission?
- results in the formation of two oppositely charged ions
- occurs when one atom retains both electrons from the sigma covalent bond and
the bond breaks unevenly - normally occurs when polar covalent bonds are broken
What do reactions involving homolytic fusion tend to result in?
The formation of very complex mixtures of products, making them unsuitable for organic synthesis
What do reactions involving heterolytic fusion tend to result in?
Far fewer products than reactions involving homolytic fission, and are better suited for organic synthesis
Which type of bond fission is better suited for organic synthesis?
Heterolytic fission
How can the movement of electrons during bond fission be represented?
Using curly arrow notation
In reactions involving heterolytic bond fission, how are attacking groups classified?
Either as electrophiles or nucleophiles
What are nucleophiles?
Negatively charged ions or neutral molecules that are electron rich
What are examples of nucleophiles?
Cl- Br- OH- CN- NH3 H2O
What are nucleophiles attracted towards?
Atoms bearing a partial or full positive charge
What are nucleophiles capable of doing?
Donating an electron pair to form a new covalent bond
What are electrophiles?
Positively charged ions or neutral molecules that are electron deficient
What are examples of electrophiles?
H+
NO2+
SO3
What are electrophiles attracted towards?
Atoms bearing a partial or full negative charge
What are electrophiles capable of doing?
Accepting an electron pair to form a new covalent bond
What are haloalkanes?
Substituted alkanes in which one or more of the hydrogen atoms is replaced with a halogen atom
How many halogen atoms does a monohaloalkane have?
One
How can monohaloalkanes be classified?
As primary, secondary or tertiary according to the number of alkyl groups attached to the carbon atom containing the halogen atom
How can monohaloalkanes be classified?
As primary, secondary or tertiary according to the number of alkyl groups attached to the carbon atom containing the halogen atom
How can monohaloalkanes become alkenes?
Through elimination reactions using a strong base, such as, potassium or sodium hydroxide in ethanol
What are the 3 nucleophilic substitution reactions monohaloalkanes can undergo?
- form alcohols with aqueous alkalis
- form ethers with alcoholic alkoxides
- form nitriles with ethanolic cyanide (chain length increased by 1 carbon atom),
these nitriles can then be hydrolysed to carboxylic acids
What are the 2 mechanisms that monohaloalkanes can undergo nucleophilic substitution reactions by?
- Sn1
- Sn2
What is Sn1?
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?
A nucleophilic substitution reaction with two species in the rate determining step and occurs in a single step via a single five-centered, trigonal bipyramidial transition state
What can be used to explain which mechanism (Sn1 or Sn2) will be preferred for a given haloalkane?
Steric hindrance and inductive stabilisation
What are alcohols?
Substituted alkanes in which one or more of the hydrogen atoms is replaced by a hydroxyl functional group (-OH group)
What are the 3 ways in which an alcohol can be prepared?
- from haloalkanes by substitution
- from alkenes by acid-catalysed hydration (addition)
- from aldehydes and ketones by reduction using a reduction agent such as lithium
aluminium hydride
How can alcohols become alkenes?
Through dehydration reactions using aluminum oxide, concentrated sulfuric acid or concentrated phosphoric acid
How can alcohols become aldehydes and then carboxylic acids?
Through oxidation of primary alcohols to form aldehydes and then further oxidation to form carboxylic acids
How can alcohols form ketones?
Through oxidation of secondary alcohols to form ketones, using acidified potassium permanganate, acidified dichromate or hot copper (II) oxide
How can alcohols become alcoholic alkoxides and then ethers?
By reaction with some reactive metals such as potassium or sodium. This alcoholic alkoxide can then be reacted with a monohaloalkane to form an ether
What are the two ways in which an alcohol can form an ester, and which is the more effective way?
- by reaction with carboxylic acids using concentrated sulfuric acid or concentrated phosphoric acid as a catalyst
- by reaction with acid chlorides
Using acid chlorides is more effective as it gives a faster reaction and does not require a catalyst
What part of a alcohol makes it polar?
The hydroxyl group, which gives rise to hydrogen bonding
What properties of alcohols can hydrogen bonding be used to explain?
- boiling points
- melting points
- viscosity
- miscibility in water
What can ethers be regarded as?
Substituted alkanes in which a hydrogen atom is replaced with an alkoxy functional group (-OR group)
How can ethers be prepared?
In a nucleophilic substitution reaction by reacting a monohaloalkane with an alkoxide
Why do ether molecules have a low boiling point compared to corresponding isomeric alcohols?
Due to the lack of hydrogen bonding between ether molecules
What are the two ethers which are soluble in water?
- methoxymethane
- methoxyethane
Why are larger ethers insoluble in water?
Due to their increased molecular size
What are ethers commonly used as and why?
They are commonly used as solvents since they are relatively inert chemically and will dissolve many organic compounds
What are the two ways in which alkenes can be prepared?
- dehydration of alcohols using aluminium oxide, concentrated sulfuric acid or concentrated phosphoric acid
- base-induced elimination of hydrogen halides from monohaloalkanes
What are the 4 electrophilic addition reactions alkenes can take part in?
- react with hydrogen to form alkanes in the presence of a catalyst
- react with halogens to form dihaloalkanes
- react with hydrogen halides to form monohaloalkanes
- react with water using an acid catalyst to form alcohols
What does Markovnikov’s rule state?
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?
What the major and minor products formed during a reaction of a hydrogen halide or water with an alkene will be
What are the 3 ways in which a carboxylic acid can be prepared?
- oxidising primary alcohols using acidified permanganate, acidified dichromate and hot copper (II) oxide
- oxidising aldehydes using acidified permanganate, acidified dichromate , Fehling’s solution and Tollen’s reagent
- hydrolysing nitriles, esters or amines
What are the 4 reactions that carboxylic acids can undergo?
- 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?
Organic derivatives of ammonia in which one or more hydrogen atoms of ammonia has been replaced by an alkyl group
How can amines be classified?
A primary, secondary or tertiary according to the number of alkyl groups attached to the nitrogen atom
What do amines form when they react with acids?
Salts
Why do primary and secondary amines have higher boiling points than isomeric tertiary amines?
Because primary and secondary amines display hydrogen bonding unlike tertiary amines
All amines are soluble, however are long or short chain amines more soluble?
Short chain amines are more soluble
How are amines similar to ammonia?
They are both 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 is benzene (C6H6)?
The simplest member of the class of aromatic hydrocarbons
What is the stability of the benzene ring 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
How can bonding in benzene be described in terms of?
sp2 hybridisation, sigma and pi bonds, and electron delocalisation
What is a phenyl group?
A benzene ring in which one hydrogen atom has been substituted by another group. It has the formula C6H5-
What type of reactions can benzene take part in?
Electrophilic substitution
What are 4 electrophilic substitution reactions that benzene can undergo?
- halogenation by reaction with 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
