Flashcards in 10.3 Types Of Organic Reactions Deck (41):
What are the two types of mechanism for nucleophilic substitution reactions?
SN1 and SN2
What does the type of mechanism that occurs depend on?
Whether the halogenoalkane is primary, secondary or tertiary.
What does SN2 mean?
Bimolecular, two species involved in the rate determining step.
What does SN1 mean?
Unimolecular, one molecule involved in the rate determining step.
What do curly arrows show in the diagram of the mechanism?
The movement of a pair of electrons.
What do fishhook arrows show in the diagram of free radical substitution?
The movement of one electron.
What happens in the SN2 mechanism?
The molecule inverts as the nucelphile attacks so that id the original halogenoalkane is chiral the reaction will occur with the inversion of the configuration at the C attached to the halogen. This type of reaction is termed stereospecific (the stereochemistry can be predicted from that of the starting material).
What type of halogenoalkanes undergo SN2 mechanism?
What type of halogenoalkanes undergo SN1 mechanism?
What happens in the SN1 mechanism?
The first step involves heterolytic fission of the C-Br bond the bond breaks so that both electrons go back at the same atom. Because the carbocation formed is planar the nucleophile can attack from either side in the second step and the reaction is not stereospecific if we start with an optically active halogenoalkane is a racemic mixture will be formed.
What is the order of an SN1 mechanism?
First order overall.
What is the order of an SN2 mechanism?
Second order overall.
What is the effect of doubling the concentration of OH- on an SN1 mechanism?
No effect as OH- is not in the rate equation rate = k[(CH3)3CBr], it is only involved in a fast step after the rate determining step.
What is the effect of doubling the concentration of OH- on an SN2 mechanism?
Rate of reaction doubles as the reaction is first order with respect to OH-
rate = k[CH3CH2Br][OH-] OH- is involved in the rate determining step.
What is important about the mechanism secondary halogenoalkanes undergo during nucleophilic substitution?
Via a mixture mechanisms, the more dominant mechanism will depend on the conditions.
What is the effect of structure on the rate of SN2 nucleophilic substitution?
The rate of reaction for SN2 is: primary > secondary > tertiary
Why is tertiary the slowest rate of reaction?
Mainly due to stern effect, more alkyl groups surrounding the central C in a halogenoalkane makes it more difficult for the nucleophile to attack the central C.
What is the effect of structure on the rate of SN1 nucleophilic substitution?
The rate of reaction for SN1 is: tertiary > secondary > primary
Why is primary the slowest rate of reaction?
Due to the stability of the intermediate carbocation. Alkyl groups have an electron releasing effect (positive inductive effect) so that they are able to stabilise a positive charged carbon atom to which they are bonded. More alkyl groups resist in a greater electron releasing effect and greater stabilisation of the carbocation.
What is the effect of the halogen on the rate of nucleophilic substitution?
R-I > R-Br > R-Cl > R-F
For both mechanism the rate determining step involved breaking the carbon halogen bond. The C-I bond is weakest therefore the reaction will be fastest for an iodoalkane.
Why is OH- a better nucleophiles than water?
Because OH- is negatively charged and will be more strongly attracted to the positive C. Therefore the rate of an SN2 reaction is slower when OH- is replaced when water as a nucleophile.
Why will changing the nucleophile have no effect on an SN1 reaction?
The nucleophile is only involved in the mechanism after the rate determining step, the nucleophile is not in the rate equation.
How are polar solvents classified?
Protic solvents - have H joined to N or O and can participate in hydrogen bonding, such as water, ethanol or ammonia.
Aprotic solvents - do not have H joined to N or O and cannot participate in hydrogen bonding, these solvents do not acts as proton donors eg propanone.
Why are SN1 mechanisms favoured by protic polar solvents?
Because solvent molecules are able to solvate both positive and negative ions which are generated in the reaction.
Why are SN2 mechanisms favoured by aprotic polar and non polar solvents?
Because these are not very good at solving the negatively charged nucleophile this means that the nucleophile is not surrounded by the solvent molecules and is better able to get in and attack the positive carbon atom. For the same reason SN2 reactions are favoured by non polar solvents.
What is an electrophile?
Species which is attached to regions of high electron density and accepts a pair of electrons to form a covalent bond. Electrophiles are lewis acids.
How do alkenes react in an electrophilic addition mechanism with a halogen?
A halogen is non polar but as it approached the C=C which is a region of high electron density a dipole is induced in the halogen (positive is closer tot he C=C). With inter halogen compounds such as I-Cl consider the difference in the electronegativity to work out which is positive and attacks the C=C.
How do alkenes react in an electrophilic addition mechanism with an interhalogen?
With inter halogen compounds such as I-Cl consider the difference in the electronegativity to work out which is positive and attacks the C=C.
How do unsymmetrical alkenes react in an electrophilic addition mechanism with a hydrogen halide or interhalogen compounds?
More than one product can be formed depending on which way the atoms add across the double bond.
What rule can be used to predict the major product from the electrophilic addition?
Markovnikov's rule - When H-X adds to the double bond of an alkene the H atom becomes attached to the C atom that has the larger number of H atoms already attached.
How can Markovnikov's rule be explained?
This can be explained in terms of the stability of the carbocation intermediates formed. The secondary carbocation is more stable than the primary carbocation because there are more electron releasing ally groups attached tot he positively charged C in the secondary carbocation.
What's important about inter halogen compounds with this rule?
The less electronegative halogen atom behaves as an H atom as it is positive and adds to the C atom with more hydrogens already attached.
What reaction does benzene undergo?
Electrophilic substitution reactions.
Why is substitution more favourable than addition for benzene?
The ring of six delocalised electrons is a region of high electron density and attracts electrophiles. Substitution is more favourable than addition as the more stable delocalised aromatic system is preserved.
When does benzene undergo a nitration reaction?
In the presence of mixture of concentrated nitric and sulphuric acid.
What are reducing agents to reduce aldehydes, ketones and carboxylic acids?
NaBH4 sodium borohydride - for aldehydes and ketons.
LiAlH4 lithium aluminium hydride - for carboxylic acids.
How are aldehydes reduced?
To primary alcohols.
How are ketones reduced?
To secondary alcohols.
What kind of reducing agent is needed for carboxylic acids?
The stronger reducing agent LiAlH4
What are carboxylic acids reduced to?
Primary alcohols - the reduction cannot be stopped at the aldehyde stage.