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Chemistry AS Level CIE > Chapter 16 Halogen compounds > Flashcards

Chapter 16 Halogen compounds Flashcards

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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

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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
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3
Q

Free-radical substitution reactions of alkanes produce halogenoalkanes example

A
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4
Q

Electrophilic addition

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  • 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
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5
Q

Electrophilic addition of hydrogen halides or hydrogen at room temperatures to alkenes results in the formation of halogenoalkanes example:

A
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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
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7
Q

Substitution of alcohols to produce halogenoalkanes example

A
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8
Q

Overview of the different ways to produce halogenoalkanes

A
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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
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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
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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
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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
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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
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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
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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
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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
Q

what do secondary halogenalkanes underg SN1 or SN2

A

Secondary halogenoalkanes undergo a mixture of both SN1 and SN2 reactions depending on their structure

26
Q

Reactivity of Halogenoalkanes

A
  • 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
Q

During substitution reactions the C-I bond will therefore

A

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
Q

Aqueous silver nitrate with halogenalkane

A
  • 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

Chemistry AS Level CIE (21 decks)

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