Halogenoalkanes and Alcohols - · PDF fileQuestions P199/201. ... R-X with ethanolic KOH Questions P203. Alcohols General Formula: C n H 2n+1 OH ... Halogenoalkanes and Alcohols Created

Haloalkanes

The basics: The functional group is a halogen atom: F, Cl, Br or I General formula – CnH2n+1 X Use the prefixes: fluoro, chloro, bromo and iodo.

Isomers: Draw and name the possible isomers for C5H11Br

Primary, secondary and tertiary haloalkanes

Haloalkanes can be classified according to the environment of the carbon atom that they are attached to.

With primary haloalkanes, the adjacent carbon atom is attached to 2 (or 3) hydrogen atoms and alkyl group (or not). In effect, the halogen is on the end of the chain

NB. If more than one halogen is present, they are written in alphabetical order.

With secondary haloalkanes, the adjacent carbon atom is attached to 1 hydrogen atom, and 2 other alkyl groups:

With tertiary haloalkanes, the adjacent carbon atom is attached to 3 alkyl groups:

Reactions of halogenoalkanes

1) With KOH

(b) Conditions: Heat under reflux with ethanol as solvent. Reaction type: elimination (OH- is acting as a base)

CH3CH2Br CH2CH2 + HBr

(a) Conditions: Heat under reflux* in aqueous solution. Reaction type: nucleophilic substitution (which species is the Nu:?)

C2H5Br + NaOH (aq) C2H5OH + NaBr

Heating under reflux enables a mixture including volatile materials to be heated for a long time without loss of solvent.

The system is designed to keep materials in the flask

2) With potassium cyanide (Nu:?) Reaction type: nucleophilic substitution Conditions: Reflux solution of halogenoalkane and potassium cyanide in ethanol.

This adds a carbon atom to the chain and forms a nitrile.

C2H5Br + KCN C2H5CN + KBr propanenitrile

PQ2.2.7

3) With ammonia Reaction type: nucleophilic substitution Nu:? Conditions: Amines are formed by heating the halogenoalkane with concentrated ammonia in a sealed tube.

C2H5I + NH3 C2H5 NH2 + HI Ethylamine (amine)

In the sealed container other products include: (C2H5)2NH and (C2H5)3N

Read 198/199 Q1-2

Test for halogenoalkane

Heat sample of halogenoalkane with aqueous silver nitrate in ethanol.

A white precipitate soluble in dilute aqueous ammonia, indicates chloride

A yellow precipitate insoluble in concentrated aqueous ammonia indicates iodide.

A buff precipitate insoluble in dilute aqueous ammonia but soluble in concentrated aqueous ammonia, indicates bromide.

The time taken for the precipitate to appear can be used as an indication of the rate of reaction…

CORE PRACTICAL 4: Investigation of the rates of hydrolysis of some halogenoalkanes

Rates of reaction of haloalkanes

Rate of hydrolysis (Nu:?) 1-iodobutane > 1-bromobutane > 1-chlorobutane

Due to the relative strengths of the C-X bond.

Bond : C-I C-Br C-Cl C-F Bond enthalpy terms (kJ mol-1 ): +238 +276 +338 +484 Thus the ease of bond breaking is, C-I > C-Br > C-Cl > C-F.

Also (U5): 3º > 2º > 1º Also (U5): 3º > 2º > 1º

Problems: Describe how to change 1-bromopropane into propan-1-ol and propene.

What is formed when 1-bromobutane is heated under reflux with alcoholic potassium cyanide?

Write equations to show all of the products which form when 1-iodopropane reacts with ammonia.

Describe how you could distinguish between a sample of 1-chloropropane and 1-bromopropane.

Questions P199/201

Mechanisms…

1. R-X with ammonia

Mechanisms…

2. R-X with aqueous KOH

3. R-X with ethanolic KOH

Questions P203

Alcohols

General Formula: Cn H 2n+1OH

Draw and name the isomers when n = 5:

Production

- Fermentation

- From alkenes

Note: H-bonding increases mpts and bpts, and increases solubility in water. Small chain alcohols H-bonding is more significant. As the chain lengthens, the “aliphatic” part of the molecule begins to dominate. Acetone is an excellent solvent for the longer chain alcohols as is is a polar organic solvent (it can H-Bond!!).

Primary alcohols Just as with haloalkanes, in a primary (1°) alcohol, the carbon which carries the -OH group is only attached to one alkyl group and 2 (or 3) H atoms: Some examples of primary alcohols include:

Secondary alcohols The adjacent carbon is attached to two alkyl groups. Examples:

Tertiary alcohols The adjacent carbon atom is attached directly to three alkyl groups. Examples:

Test for the –OH (hydroxyl) group*:

C2H5OH + PCl5 C2H5Cl + POCl3 + HCl

Or, more generally:

R-OH + PCl5 R-Cl + POCl3 + HCl

*in alcohols and carboxylic acids.

Phosphorus Pentachloride produces white steamy fumes of HCl in forming an alkyl chloride.

The reaction vessel must be perfectly dry, as water also reacts to give steamy fumes – try the equation:

NB. Terciary alcohols will also react with concentrated HCl to form chloroalkanes…

Reactions of alcoholsWith: K2Cr2O7(aq) Conc. H3PO4

NaBr or KBr P and I

With acidified K2Cr2O7 (aq) This is oxidation, and produces aldehydes, ketones and carboxylic acids (used to distinguish between 1º, 2º and 3º alcohols).

In organic chemistry, we use simplified versions of redox reactions. An oxidising agent is represented as [O].

*aldehydes and ketones have the general formula CnH2nO – they contain a C=O, aldehydes at the end of the chain and ketones somewhere along it. See later.

Primary alcohols can be oxidised first to aldehydes* and then to carboxylic acids depending on the reaction conditions.

Partial oxidation to aldehydes Excess alcohol, and distil off the aldehyde as soon as it forms (volatile).

The excess of the alcohol means that there isn't enough oxidising agent present to carry out the second stage.

Ethanol produces the aldehyde CH3CHO - Ethanal

What's happening...

The second stage:

Overall...

Full oxidation to carboxylic acids Excess oxidising agent and heat under reflux. When the reaction is complete, the carboxylic acid is distilled off.

The reaction has nowhere to go, as there is no H atom on the C=O to slip another [O] in.

Secondary alcohols are oxidised to ketones*

Eg. Heat propan-2-ol with potassium dichromate(VI) solution acidified with dilute sulphuric acid…

This reaction can be used as a test to distinguish between 1º, 2º and 3º

Tertiary alcohols Tertiary alcohols cannot be oxidised with potassium dichromate(VI) solution.

The carbon-oxygen double bond cannot be formed as the 2H atoms asociated with the C atom do not exist:

Alternatively...

Conc. H3PO4

Heat the alcohol with concentrated phosphoric(V)acid in

this dehydration reaction (no mechanism required).

*We used PCl5 to insert Cl

With NaBr or KBr and conc. H2SO4

To replace the –OH with Br*

Add to the alcohol a mixture of sodium or potassium bromide and concentrated sulphuric acid.

This produces hydrogen bromide (in situ) which reacts with the alcohol. The mixture is warmed to distil off the bromoalkane

Eg.

With P and I - Replacing -OH by iodine

Moist red phosphorus and iodine are heated with the alcohol (again, to prepare the PI in situ).

2P + 3I2 2PI3

PI3 + 3ROH H3PO3 + 3RI

Eg. How would you prepare 1-iodopropane?

Questions P205/207

EXTRA: Nomenclature of aldehydes, ketones and carboxylic acids

NB. We number from the end closest to the C=O

Synthetic pathways The reactions we have covered are fundamental in the synthesising of desired molecules in organic chemistry. Perhaps nature will provide us with a similar molecule to the one required, and we manipulate it to meet our needs.

TASK: Draw a spider diagram of all the Organic Reactions to date

INCLUDE starting materials, conditions, catalysts, etc

Separating and Purifying Organic liquids

Read PP208-211 and write notes on…

• Heating under reflux

• Solvent extraction

• Simple and fractional distillation

• Drying with an anhydrous salt

• Boiling temperature determination