Chapter 16 Halogen compounds Flashcards
1
Q
Halogenoalkanes
A
are alkanes that have one or more halogens
-They can be produced from:
–Free-radical substitution of alkanes
–Electrophilic addition of alkenes
–Substitution of an alcohol
2
Q
Free-radical substitution of alkanes
A
- Ultraviolet light (UV) is required for the reaction to start off
- A free-radical substitution reaction is a three-step reaction consisting of initiation, propagation and termination steps
- In the initiation step the halogen bond is broken by energy from the UV light to produce two radicals in a homolytic fission reaction
- The propagation step refers to the progression (growing) of the substitution reaction in a chain type reaction
- The termination step is when the chain reaction terminates (stops) due to two free radicals reacting together and forming a single unreactive molecule
3
Q
Free-radical substitution reactions of alkanes produce halogenoalkanes example
A
4
Q
Electrophilic addition
A
- Halogenoalkanes can also be produced from the addition of hydrogen halides (HX) or halogens (X2) at room temperature to alkenes
- In hydrogen halides, the hydrogen acts as the electrophile and accepts a pair of electrons from the C-C bond in the alkene
- The major product is the one in which the halide is bonded to the most substituted carbon atom (Markovnikov’s rule)
- In the addition of halogens to alkenes, one of the halogen atoms acts as an electrophile and the other as a nucleophile
5
Q
Electrophilic addition of hydrogen halides or hydrogen at room temperatures to alkenes results in the formation of halogenoalkanes example:
A
6
Q
Substitution of alcohols
A
- In the substitution of alcohols an alcohol group is replaced by a halogen to form a halogenoalkane
- The subustition of the alcohol group for a halogen can be achieved by reacting the alcohol with:
- HX (or KBr with H2SO4 or H3PO4 to make HX)
- PCl3 and heat
- PCl5 at room temperature
- SOCl2
7
Q
Substitution of alcohols to produce halogenoalkanes example
A
8
Q
Overview of the different ways to produce halogenoalkanes
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9
Q
Classifying Halogenoalkanes
A
- A primary halogenoalkane is when a halogen is attached to a carbon that itself is attached to one other alkyl group
- A secondary halogenoalkane is when a halogen is attached to a carbon that itself is attached to two other alkyl groups
- A tertiary halogenoalkane is when a halogen is attached to a carbon that itself is attached to three other alkyl groups
10
Q
Nucleophilic Substitution Reactions of Halogenoalkanes
A
- Halogenoalkanes are much more reactive than alkanes due to the presence of the electronegative halogens
- The halogen-carbon bond is polar causing the carbon to carry a partial positive and the halogen a partial negative charge
- A nucleophilic substitution reaction is one in which a nucleophile attacks a carbon atom which carries a partial positive charge
- An atom that has a partial negative charge is replaced by the nucleophile
11
Q
Nucleophilic Substitution Reactions of Halogenoalkanes: Reaction with NaOH
A
- The reaction of a halogenoalkane with aqueous alkali results in the formation of an alcohol
- The halogen is replaced by the OH–
- The aqueous hydroxide (OH– ion) behaves as a nucleophile by donating a pair of electrons to the carbon atom bonded to the halogen
- Hence, this reaction is a nucleophilic substitution
12
Q
Nucleophilic Substitution Reactions of Halogenoalkanes Reaction with KCN
A
- The nucleophile in this reaction is the cyanide, CN– ion
- Ethanolic solution of potassium cyanide (KCN in ethanol) is heated under reflux with the halogenoalkane
- The product is a nitrile
- The nucleophilic substitution of halogenoalkanes with KCN adds an extra carbon atom to the carbon chain
- This reaction can therefore be used by chemists to make a compound with one more carbon atom than the best available organic starting material
13
Q
Nucleophilic Substitution Reactions of HalogenoalkanesReaction with NH3:
A
- The nucleophile in this reaction is the ammonia, NH3 molecule
- An ethanolic solution of excess ammonia (NH3 in ethanol) is heated under pressure with the halogenoalkane
- The product is a primary amine
- It is very important that the ammonia is in excess as the product of the nucleophilic substitution reaction, the ethylamine, can act as a nucleophile and attack another bromoethane to form the secondary amine, diethylamine
14
Q
Nucleophilic Substitution Reactions of Halogenoalkanes: Reaction with aqueous silver nitrate
A
- Halogenoalkanes can be broken down under reflux by water to form alcohols
- The breakdown of a substance by water is also called hydrolysis
- This reaction is classified as a nucleophilic substitution reaction with water molecules in aqueous silver nitrate solution acting as nucleophiles, replacing the halogen in the halogenoalkane
- This reaction is similar to the nucleophilic substitution reaction of halogenoalkanes with aqueous alkali, however, hydrolysis with water is much slower than with the OH– ion in alkalis
- The hydroxide ion is a better nucleophile than water as it carries a full formal negative charge
- In water, the oxygen atom only carries a partial negative charge
15
Q
A hydroxide ion is a better nucleophile as it has a full formal negative charge whereas the oxygen atom in water only carries a partial negative charge; this causes the nucleophilic substitution reaction with water to be much slower than with aqueous alkali
A
16
Q
Halogenoalkanes: Elimination Reactions
A
- In an elimination reaction, an organic molecule loses a small molecule
- In the case of halogenoalkanes this small molecule is a hydrogen halide (eg. HCl)
- The halogenoalkanes are heated with ethanolic sodium hydroxide causing the C-X bond to break heterolytically, forming an X– ion and leaving an alkene as an organic product
17
Q
Hydrogen bromide is eliminated to form ethene
A
18
Q
Halogenoalkanes: SN1 & SN2 Mechanisms
A
- In nucleophilic substitution reactions involving halogenoalkanes, the halogen atom is replaced by a nucleophile
- These reactions can occur in two different ways (known as SN2 and SN1 reactions) depending on the structure of the halogenoalkane involved
19
Q
SN2 reactions
A
- In primary halogenoalkanes, the carbon that is attached to the halogen is bonded to one alkyl group
- The SN2 mechanism is a one-step reaction
- The nucleophile donates a pair of electrons to the δ+ carbon atom to form a new bond
- At the same time, the C-X bond is breaking and the halogen (X) takes both electrons in the bond (heterolytic fission)
- The halogen leaves the halogenoalkane as an X– ion
20
Q
The mechanism of nucleophilic substitution in bromoethane which is a primary halogenoalkane (SN2)
A
21
Q
SN1 reactions
A
- In tertiary halogenoalkanes the carbon that is attached to the halogen is bonded to three alkyl groups
- The SN1 mechanism is a two-step reaction
- In the first step, the C-X bond breaks heterolytically and the halogen leaves the halogenoalkane as an X– ion (this is the slow and rate-determining step)
- This forms a tertiary carbocation (which is a tertiary carbon atom with a positive charge)
- In the second step, the tertiary carbocation is attacked by the nucleophile
22
Q
The mechanism of nucleophilic substitution in 2-bromo-2-methylpropane which is a tertiary halogenoalkane (SN1)
A
23
Q
Carbocations
A
- In the SN1 mechanism, a tertiary carbocation is formed
- This is not the case for SN2 mechanisms as a primary carbocation would have been formed which is much less stable than tertiary carbocations
- This has to do with the positive inductive effect of the alkyl groups attached to the carbon which is bonded to the halogen atom
- The alkyl groups push electron density towards the positively charged carbon, reducing the charge density
- In tertiary carbocations, there are three alkyl groups stabilising the carbocation whereas in primary carbocations there is only one alkyl group
- This is why tertiary carbocations are much more stable than primary ones
24
Q
The diagram shows the trend in stability of primary, secondary and tertiary carbocations
A
25
what do secondary halogenalkanes underg SN1 or SN2
Secondary halogenoalkanes undergo a mixture of both SN1 and SN2 reactions depending on their structure
26
Reactivity of Halogenoalkanes
- The halogenoalkanes have different rates of substitution reactions
- Since substitution reactions involve breaking the carbon-halogen bond the bond energies can be used to explain their different reactivities
27
During substitution reactions the C-I bond will therefore
heterolytically break as follows:
- R3C-I + OH– → R3C-OH + I–
- The C-F bond, on the other hand, requires the most energy to break and is, therefore, the strongest carbon-halogen bond
- Fluoroalkanes will therefore be less likely to undergo substitution reactions
28
Aqueous silver nitrate with halogenalkane
- Reacting halogenoalkanes with aqueous silver nitrate solution will result in the formation of a precipitate
- The rate of formation of these precipitates can also be used to determine the reactivity of the halogenoalkanes
- The formation of the pale yellow silver iodide is the fastest (fastest nucleophilic substitution reaction) whereas the formation of the silver fluoride is the slowest (slowest nucleophilic substitution reaction)
- This confirms that fluoroalkanes are the least reactive and iodoalkanes are the most reactive halogenoalkanes
