Chapter 16 Halogen compounds Flashcards
Halogenoalkanes
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
Free-radical substitution of alkanes
- 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
Free-radical substitution reactions of alkanes produce halogenoalkanes example
Electrophilic addition
- 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
Electrophilic addition of hydrogen halides or hydrogen at room temperatures to alkenes results in the formation of halogenoalkanes example:
Substitution of alcohols
- 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
Substitution of alcohols to produce halogenoalkanes example
Overview of the different ways to produce halogenoalkanes
Classifying Halogenoalkanes
- 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
Nucleophilic Substitution Reactions of Halogenoalkanes
- 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
Nucleophilic Substitution Reactions of Halogenoalkanes: Reaction with NaOH
- 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
Nucleophilic Substitution Reactions of Halogenoalkanes Reaction with KCN
- 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
Nucleophilic Substitution Reactions of HalogenoalkanesReaction with NH3:
- 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
Nucleophilic Substitution Reactions of Halogenoalkanes: Reaction with aqueous silver nitrate
- 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
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
Halogenoalkanes: Elimination Reactions
- 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
Hydrogen bromide is eliminated to form ethene
Halogenoalkanes: SN1 & SN2 Mechanisms
- 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
SN2 reactions
- 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
The mechanism of nucleophilic substitution in bromoethane which is a primary halogenoalkane (SN2)
SN1 reactions
- 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
The mechanism of nucleophilic substitution in 2-bromo-2-methylpropane which is a tertiary halogenoalkane (SN1)
Carbocations
- 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
The diagram shows the trend in stability of primary, secondary and tertiary carbocations
what do secondary halogenalkanes underg SN1 or SN2
Secondary halogenoalkanes undergo a mixture of both SN1 and SN2 reactions depending on their structure
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
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
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
