15- Haloalkanes Flashcards by Maya Hennig

What are the prefix for halogens?

F- fluoro
Cl- chloro
Br- bromo
I- iodo

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What are haloalkanes?

-compounds that contain carbon, hydrogen and at least one halogen

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Are halogens polar? Why? (reactivity of halogens)

-Yes
-Halogen are more electronegative then C atoms
-The electron pair in the carbon-halogen bond is therefore closer to the halogen atom than the carbon atom causing a polar bond

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What happens to haloalkanes due to carbon atoms being slightly electropositive?

-can attract species containing a long pair of electrons

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What is a neuleophile?

-a species that can donate a lone pair of electrons
-an atom or group of atoms that is attracted to an electron deficient carbon atom, where it donates a pair of electrons to form a new covalent bond

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What are some examples of nucleophiles?

-hydroxide ions OH-
-water molecules H20
-ammonia molecules NH3

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What happens when a haloalkane reacts with a nucleophile?

-nucleophile replaces the halogen in a substitution reaction
-new compounded produced with a different functional group
-mechanism = nucleophillic substitution

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What type of haloalkanes undergo nucleophillic substitution?

Primary haloalkanes

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What is hydrolysis?

a chemical reaction involving water or an aqueous solution of hydroxide that causes the breaking of a bond in a molecule. Results in molecule being split into two products

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Explain the mechanism of nucleophillic substitution in the hydrolysis of primary haloalkanes with aqueous alkali (OH-)

The nucleophile, OH-, approaches the carbon atom attached to the halogen on the opposite side of the molecule to the halogen atom
the direction of the attack of the OH- ions minimises the repulsion between the nucleophile and electronegative halogen atom
a lone pair of electrons on the hydroxide ion is attracted and donated to the electropostive carbon atom
a new bond is formed between the the oxygen atom of the hydroxide ion and the carbon atom
the carbon-halogen bond breaks by heterolytic fission
the new organic product is an alcohol. A halide ion is also formed

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What are the conditions for halogens being converted to alcohols?

-use aqueous sodium hydroxide
-heated under reflux to obtain a good yield of product

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What is the equation for hydrolysis of
1-bromobutane?

CH3CH3CH2CH2Br + NaOH → CH3CH2CH2CH2OH + NaBr

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when a haloalkanes reacts with a nucleophile, what is the name of the reaction mechanism?

Nucleophillic substitution

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What determines how fast a specific carbon-halogen bond will break?

Bond enthalpy

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Which haloalkane will react the fastest and why?

-iodoalkanes
-because the C-I bonds are the weakest so require less energy to break

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Why are fluoroalkanes unreactive?

-strongest carbon-halogen bond
-require a large quantity of energy to break the C—F bond

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Which carbon-halogen bond is the weakest? Which is the strongest?

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C—I is the weakest (so reacts fastest)
-Then C—Br
-Then C—Cl
-C—F is the strongest (unreactive, lots of energy required to break bond)

How can hydrolysis of primary haloalkanes be compared to one another?

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-Preform the reaction in presence of aqueous silver nitrate (halide ions are produced which react with Ag + ions to form a precipitate

Ag+ + X- → AgX

-nucleophile in reaction is water which is present in the silver nitrate. Haloalkanes are insoluble in water, reaction carried out in the presence of an ethanol solvent which allows water and the haloalkane to mix and produce a single solution

What would the results of test show? (for 1-chlorobutane, 1-bromobutane, 1-iodobutane)

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-chloroalkanes react slowest
-iodoalkanes react fastest
-rate of hydrolysis increases as strength of the carbon-halogen bond decreases

How does a primary, secondary or tertiary haloalkanes status affect the rate of hydrolysis?

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-tertiary reacts fastest because the intermediary carbocation that is formed is much more stable than a primary carbocation

What are organohalogen compounds?

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molecules that contain at least one halogen atom joined to a carbon chain

What are the uses of organohalogens?

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-pesticides
-general solvents
-dry cleaning solvents
-making polymers
-flame retardants
-refrigerants

Where are organohalogen compounds found?

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-rarely found in nature as they are not broken down naturally in the environment

What is the ozone layer?

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-found at the outer edge of the stratosphere at a height that varies from around 10-40km above earths surface
-only a tiny fraction of the gases here are actually ozone

What is ozone’s function?

Absorb most of the biologically damaging ultraviolet radiation (UV-B) from the suns rays allowing only a small amount to reach earths surface

What is UV-B commonly linked to?

-sunburn -genetic damage -risk of skin cancer

Why is the depletion of the ozone layer feared?

-more UV-B will be allowed to the earths surface -leading to increased genetic damage and greater risk of skin cancer in humans

How is ozone being continually formed and broken?

-by the action of ultraviolet radiation -very high UV breaks oxygen molecules into oxygen radicals O: O2 → 2O -steady state set up involving O2 and oxygen radicals in which ozone breaks and forms -in steady state the rate of formation of ozone is equal to the rate at which it is breaking down O2 + O ⇌ O3

What has upset the steady state of ozone formation and breaking down?

-chlorofluorocarbons (CFCs)

What is the impact of CFC’s on earths atmosphere?

-CFCs remain stage until they reach the stratosphere where they begin to break down and form chlorine radicals which are thought to catalyse the breakdown of the ozone layer

how do CFCs deplete the ozone layer?

UV ration provides sufficient energy to break a carbon-halogen bond in the CFCs by homolytic fission to form radicals (C—Cl bond has lowest bond enthalpy so is the bond that breaks

What is photodissociation? What is the photodissociation of CF2Cl2?

-photodissociation is where radiation initiates the breakdown CF2Cl2 → CF2Cl• + Cl•

What is the propagation steps for Cl radicals reacting with ozone?

Step 1: Cl• + O3 → ClO• + O2 Step 2: ClO• + O → Cl• + O2 Overall equation: O3 + O → 2O2 Propagation step 2 regenerates a chlorine radical, which can attack and remove another molecule of ozone in propagation step one. The two propagation steps repeat in a cycle over and over again in a chain reaction.

What is the estimate if how many ozone molecules 1 CFC molecule can promote the breakdown of?

1 CFC molecules promotes the breakdown of 100,000 ozone molecules

are CFCs responsible for all ozone-depleting reactions?

No, other radicals also catalyse the breakdown of ozone.

How are nitrous oxide radicals formed?

-naturally during lightining strikes -as a result of aircraft travel in the stratosphere

What are the propagation steps for nitrous oxide radicals breaking down ozone?

Step 1: NO• + O3 → NO2• + O2 Step 2: NO2• + O → NO• + O2 Overall equation: O3 + O → 2O2

What is a radical?

a species with an unpaired electron

When halide ion is reacted with silver nitrate what colour precipitate do they form?

chlorine - white bromine - cream iodine - yellow

What does a curly arrow represent in mechanisms?

movement of an electron pair

Are haloalkanes soluble in water?

No, they are insoluble as C-H bonds are nonpolar, no compensated for enough by C-X bond polarity

When the hydrolysis of a primary haloalkane is tested what colour precipitate is formed for each corresponding halogen?

chlorine - white bromine - cream iodine - yellow

15- Haloalkanes Flashcards

(42 cards)