Reactions in Organic Chemistry - Substitution Reaction Flashcards Preview

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Flashcards in Reactions in Organic Chemistry - Substitution Reaction Deck (22)
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1

Substitution reaction

A chemical reaction in which an atom/group of atoms in a molecule is replaced by another atom/group of atoms

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example of substitution reaction

Alkanes reaction w/ halogens in presence of ultraviolet light

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Monochlorination

replacing just one atom of hydrogen by one atom of chlorine (called monochlorination of methane)

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HCl

not hydrochloric acid in this, it is hydrogen chloride

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Mechanism

mechanism of a reaction is the detailed step-by-step description of how the overall reaction occurs

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mechanism of reaction use

needed in order to establish the best conditions of a particular reaction ( to maximise yield of products)

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chemical reactions

involve bond breaking + bond formation.
When bonds broken/made, there is always a rearrangement of electrons

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curly arrows

half of head of arrow - movement of 1 electron

full of head of arrow - movement of electron pair

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mechanism of monochlorination of methane - steps

1. initiation
2. propagation
3. propagation
4. termination

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mechanism of monochlorination of methane -
Step 1: initiation

1.Chlorine molecule broken down into two chlorine atoms in prsence of uv light. Uv light provides energy to break bond in chlorine molecule + convert chlorine molecule into two chlorine free readicals

2. Retains one of 2 electrons from bond. This type of bond breaking is called homolysis/homolytic fission

3. Key feature is that each of two products ends up w/ one of pair of electrons involved in bonding togethe

4. This reaction is example of photochemical reaction.

-evidence of photochemical reaction

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mechanism of monochlorination of methane -
Evidence for photochemical reaction

Evidence comes from fact reaction doesnt occur in dark at room temp
(photochemical reaction)

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mechanism of monochlorination of methane -
Step 2: propagation

1. Chlorine atom attacks methane molecule to form hydrogen chloride + species called methly free radical (any atom/group of atoms w unpaired electron. Symbol is as dot)

2. Propagation involves use of free radical to produce another free radical. Since they do not have full octet of electrons in outer energy level, are v. reactive + usually only exist for v. short periods.

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mechanism of monochlorination of methane -
Step 3: propagation

1. Methyl free radical attacks chlorine molecule to form chloromethane + a chlorine atom

2. Chlorine atom can react w/ another molecule of methane. Sets up chain reaction (continues on bc a product from one step is reactant for another step)

-chain reaction evidence

3. Found that for every photon absorbed, many thousands of molecules of chloromethane were produced

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mechanism of monochlorination of methane -
Chain reaction evidence

Evidence for this chain reaction comes from a study of the amount of light absorbed by the glass container in which reaction is taking place.

Found that for every photon absorbed, many thousands of molecules of chloromethane were produced.

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mechanism of monochlorination of methane -
Step 4: termination

1. Number of reactions take place to stop chain reaction. Involve free radicals.

2. Reaction comes to an end in this stage.

3. Chain reaction can go thru hundreds/thousands cycles before termination. Reactions extremely fast + reaction has explosive nature

-evidence of free radicals combining

4. Tetramethyl lead inc. rate of reaction

5. Chlorination of methane is commonly referred to as a free radical substitution reaction

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mechanism of monochlorination of methane -
Combining of free radicals evidence

Evidence for combining of free radicals comes from fact that small amounts of ethane are found among the products. Ethane must have come from the combination of two methyl free radicals.

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Mechanism for the chlorination of ethane

Same mech holds for chlorination of ethane as chlorination of methane

Radical formed is ethyl radical

termination stage - compound formed will be butane

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Uses of fully halogenated alkanes

Fully halogenated alkanes are added to fabrics as flame retardants

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Esterification - what its also called

Called a condensation reaction since water formed in reaction. Also substitution since H attached to -OH group is carboxylic acid molecule has been replaced by an alkyl group

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Esterification

Formation of an ester, an equilibrium reaction

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reverse esterification

hydrolysis

Esters are hydrolysed v. effectively in presence of a base

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Hydrolysis of esters

-forms the sodium salt of the carboxylic acid rather than the carboxylic acid itself

-used in manufacture of soap (saponification)

-Catalytic step involves the OH⁻ ions from NaOH + results in production of acid + alcohol

-Acid then donates H⁺ to the OH⁻ ions forming water + carboxylate ions