3.3.3 Halogenoalkanes Flashcards by Mariam Ahmad

What is a halogenoalkane?

Alkanes with at least one halogen atom in place of hydrogen atom

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Describe 3 physical properties of halogenoalkanes

Insoluble in water ∵ polar C^δ+—X^δ- bonds are not polar enough
Main intermolecular forces of attraction are dipole—dipole attraction and van der Waals forces
Halogenoalkanes mix with hydrocarbons

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Boiling points increase with … & increases when …

Boiling points increase with increased chain length & increases when going down the halogen group

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Why do boiling points increase with increased chain length or when going down the halogen group?

∵ larger the molecule = greater the number of electrons ∴ greater the van der Waals forces

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Why do halogenoalkanes have higher boiling points than alkanes with similar chain lengths?

∵ have higher relative molecular masses & more polar

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Why are carbon-halogen bonds polar?

Halogens are much more electronegative than carbon

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What makes halogenoalkanes prone to attacks from nucleophiles?

δ+ charge on carbon/polar carbon-halogen bond

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What is a nucleophile?

Electron-pair donor

(negatively charged ion or an atom with δ- charge)

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What do nucleophiles have?

Has lone (unshared) pair of electrons

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As you go down the halogen group, the bonds (C-X) get __ ___

less polar

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What is a nucleophilic substitution reaction?

When a nucleophile reacts with polar molecules by replacing the functional group

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Why are nucleophilic substitution reactions used?

To introduce new functional groups into organic compounds

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Nucleophilic Substitution

Name the reactant

Halogenoalkane

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Nucleophilic Substitution

Name the nucleophiles

:OH^-, :CN^- & :NH₃

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Nucleophilic Substitution

Name the condition & product for the nucleophile :OH^-

Warm, aqueous NaOH or KOH → Alcohol

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Nucleophilic Substitution

Name the condition & product for the nucleophile :CN^-

Warm halogenoalkane with ethanolic KCN_(aq) → Nitrile

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Nucleophilic Substitution

Name the condition & product for the nucleophile :NH₃

Warm halogenoalkane with excess ethanolic ammonia → Amine

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Why is the reaction between hydroxide ions & halogenoalkanes sometimes called hydrolysis?

∵ in the mechanism, the exact same reaction will happen with water

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Bromoethane + aqueous NaOH

Name the mechanism

Nucleophilic Substitution

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Bromoethane + aqueous NaOH

Balanced Equation

C₂H₅Br + NaOH → C₂H₅OH + NaBr

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Bromoethane + aqueous NaOH

Name of Organic Product

Ethanol

Bromoethane + aqueous NaOH

Mechanism

Bromoethane + NH₃

Name of Mechanism

Nucleophilic Substitution

Bromoethane + NH₃

Balanced Equation

Bromoethane + NH₃ Mechanism

Bromoethane + NH₃ Name of Organic Compound

Ethylamine

Bromoethane + KCN Name of Mechanism

Nucleophilic Substitution

Bromoethane + KCN Mechanism

Bromoethane + KCN Balanced Equation

C₂H₅Br + KCN → C₂H₅CN + KBr

Bromoethane + KCN Name of Organic Compound

Propanenitrile

When halogenoalkanes react with ammonia, what can the amine group in the product act as and why & what can this lead to?

Can acts as nucleophile∵ it has lone pair of electrons ∴ may react with halogenoalkane molecules = giving mixture of products

Why is an excess of ammonia needed to react with a halogenoalkane to produce a high yield?

1. The amine group in the product = nucleophile ∴ could react further & make secondary/tertiary amines OR 2. Ammonia acts both initially as a nucleophile and then as a base

What 2 factors affect the carbon-halogen bond's reactivity?

* C^δ+—X^δ- bond polarity * C—X bond enthalpy (i.e. strength)

Bond Enthalpies Bonds get ____ going down the group

Bonds get **weaker** going down the group

Explain why fluorine has the strongest bond

It's the smallest atom of halogens & shared electrons in C—F bond = strongly attracted to nucleus

Explain why bonds get weaker going down the group

∵ shared electrons in C—X bond gets further away from halogen nucleus = bond becomes weaker

What does the polarity of C—X predict & why?

* C—F most reactive * ∵ its most polar - C^δ+ has most positive charge ∴ more easily attacked by nucleophile * C—I = least reactive * ∵ it's least polar

What is the main factor that decides reactivity?

Carbon-halogen bond enthalpy (Experiments show that reactivity increases down the group ∵ bond enthalpy is more important factor than bond polarity)

Why do fluoroalkanes undergo nucleophilic substitution reactions more slowly than other halogenoalkanes?

C-F bond is strongest = highest bond enthalpy

Why are iodoalkanes substituted more quickly than say fluoroalkanes?

C-I bond has lowest bond enthalpy = easier to break

When do you get an elimination reaction?

If you warm a halogenoalkane with hydroxide ions dissolved in ethanol instead of water = elimination reactions happens

**Elimination Reaction** Name the condition and product when a halogenoalkane is used as a rectant

Ethanolic KOH - Alkene

2-bromopropane + ethanolic KOH Name of Mechanism

Elimination Reaction

2-bromopropane + ethanolic KOH Balanced Equation

C₃H₇Br + KOH → C₃H₆ + H₂O + KBr

2-bromopropane + ethanolic KOH Name of Organic Compound

Propene

2-bromopropane + ethanolic KOH Mechanism

In an elimination reaction, why do you have to heat mixture under reflux?

∵ you'll otherwise lose the volatile stuff

In elimination reactions: small group of atoms....

break away from molecule Group is NOT replaced

When halogenoalkanes are reacted with hydroxides, they can undergo...

either nucleophilic substitution or elimination → 2 reactions are said to be competing

Identify & draw an isomer of 2-bromopentane that does not react with hot, ethanolic sodium hydroxide to produce an alkene

1-bromo-2,2-dimethylpropane

Suggest one change to the reaction conditions that would increase the yield of an alcohol (Hot concentrated ethanolic potassium hydroxide reacts with halogenoalkane)

* Use water as a solvent (aq conditions) * Lower temperature * Dilute KOH

Suggest one change to the reaction conditions that would increase the yield of an alkene

* Ethanolic solvent * High temperature * High concentration of KOH