Chemistry - Topic 17 - Organic II

what are optical isomers?

optical isomers are non-superimposable mirror images of each other

what is optical activity?

optical activity is the ability of a single optical isomer to rotate the plane of polarisation of plane-polarised monochromatic light in molecules containing a single chiral centre

why is there no optical activity observed from a racemic mixture of optical isomers?

optical isomers rotate the plane of plane-polarised light in opposite directions (therefore cancelling out rotational effects)

does a primary halogenoalkane go via S_N1 or S_N2? If reactant is chiral, what is the product?

• S_N2

• if we start with a single enantiomer, the product is a single enantiomer but with the stereochemistry inverted

does a tertiary halogenoalkane go via S_N1 or S_N2? If reactant is chiral, what is the product?

• S_N1

• if we start with a single enantiomer of a tertiary halogenoalkane, the product is a racemic mixture. this implies two different mechanisms are taking place

why is stereochemistry inverted during S_N2?

• two species involved in the slowest step

• the nucleophile must attack from 180 degrees away from the leaving group in an S_N2 mechanism. this leads to inversion of stereochemistry.

why is a racemic mixture formed via an S_N1 mechanism?

• trigonal planar around the central carbon atom

• the nucleophile can attack from above or below the plane of the carbocation with 50:50 probability leading to a racemic mixture

why do tertiary halogenoalkanes undergo S_N1 mechanisms?

• tertiary carbocations are more stable than secondary, which are more stable than primary

• therefore the C-Hal bond in a tertiary halogenoalkane is the weakest and most likely to break to form a carbocation. tertiary halogenoalkanes are also unlikely to do an S_N2 mechanism because the delta positive carbon is crowded with alkyl groups so it is sterically difficult for the nucleophile to attack

what reducing agent and conditions can be used to reduce a carbonyl (aldehyde or ketone). what type of mechanism and reaction is this? what is formed?

• dry lithium aluminium hydride (LiAlH₄), aka lithal or dry sodium borohydride (NaBH₄)

• in dry ether

• followed by dilute HCl(aq) to protonate the intermediate formed (acidic work up)

• if aldehyde or ketone is reacting then this is a nucleophilic addition (and a reduction), where the hydride ion, H^- acts as the nucleophile

• if a carboxylic acid is reacting then this is just a reduction

• an alcohol is formed

how to test for a carbonyl?

• 2,4 - DNPH

• positive test → red/orange/yellow crystals (it is a spectrum) - often yellow ppt on PPQs

• this is a qualitative test

specific to PPQs:

• add 2,4-DNPH and collect precipitate by filtration

• recrystallise

• measure melting point and compare with a data base

how do you identify a carbonyl compound?

take melting point of crystals and compare with data base values to identify the aldehyde/ketone

what type of mechanism and reaction occurs when a carbonyl reacts with HCN (a weak acid) in the presence of a KCN homogenous catalyst? what type of product is formed?

• nucleophilic addition

• HCN and KCN is a buffer

• a hydroxynitrile is formed

why can nucleophilic addition lead to a racemic mixture as a product?

the carbonyl is trigonal planar about the C=O. therefore, the cyanide ion nucleophile can attack from above or below the plane with 50:50 probability

why will nucleophilic addition involving a carbonyl and HCN in the presence of KCN not occur at a low pH?

• only HCN

• (K⁺)

• no cyanide ion as it has been protonated therefore no nucleophile

• at low pH there are not enough deprotonated cyanide ions to act as a nucleophile in the first step/[CN^-] is too low therefore rate would be too slow

why will nucleophilic addition involving a carbonyl and HCN in the presence of KCN not occur at a high pH?

• at a high pH, there is not enough HCN/H⁺ to protonate the intermediate in the second step

• too low [H⁺]

how to test for a methyl next to a carbonyl? give an example equation for testing for a methyl group in propanone with state symbols.

• reactants: iodine and aqueous sodium hydroxide (alkali)

• positive test → yellow precipitate (CHI₃ - triiodomethane, also known as iodoform) + antiseptic smell

• also produces a salt of carboxylic acid

• CH₃COCH₃(aq) + 3I₂(aq) + 4NaOH(aq) → CH₃COO^(-)Na⁽⁺⁾(aq) + CHI₃(s) + 3Na⁽⁺⁾I^(-)(aq) + 3H₂O(l)

what determines whether a hydroxide ion is a base or a nucleophile?

• base → forms bond with H⁺

• nucleophile → forms bond with delta positive carbon atom

state two methods in which you can increase the length of a carbon chain?

• reacting a carbonyl with HCN in the presence of KCN (not efficient as can only add one carbon at a time)

• using magnesium and dry ether to form a Grignard reagent (compound containing MgBr in dry ether), then react aldehyde with Grignard followed by an acidic workup (source of H⁺ ions)

• reacting a compound containing a Br with magnesium makes delta positive carbon atom become delta negative, so that MgBr can break off and the carboanion can attack delta positive carbon atom of carbonyl

how to create a carboxylic acid from a Grignard? what type of mechanism and reaction is this? give an example balanced equation starting with C₆H₅MgBr (phenyl magnesium bromide).

react carbon dioxide (dry ice) with a Grignard

name eight ways to prepare a carboxylic acid.

• complete oxidation of a primary alcohol

• oxidation of an aldehyde

• acid hydrolysis of a nitrile

• alkaline hydrolysis of a nitrile followed by an acidic work up

• reacting carbon dioxide with a Grignard reagent

• reacting an acyl chloride with water

• acid hydrolysis of an ester

• acid-base reaction with a carboxylate salt

how do you form a carboxylic acid from a nitrile? state conditions and an example balanced equation starting with ethane nitrile.

• acid hydrolysis

• heat/H⁺(aq) - heat under reflux with dilute HCl(aq)

• CH₃CN + 2H₂O + HCl → CH₃COOH + NH₄Cl

  OR

• alkaline hydrolysis

• heat under reflux

• reactants: NaOH(aq) followed by dilute acidic work up with HCl(aq)

• CH₃CN + H₂O + NaOH → CH₃COO⁻Na⁺ + NH₃

• CH₃COO⁻Na⁺ + H⁺ → CH₃COOH + Na⁺ (check)

how do you convert a nitrile into a carboxylic salt? state conditions and an example word equation and balanced equation starting with ethanenitrile.

• hydrolysis of nitrile under alkali conditions - alkaline hydrolysis

• heat under reflux with NaOH(aq) (base)

• e.g. ethanenitrile + water + sodium hydroxide → sodium ethanoate + ammonia

• CH₃CN + H₂O + NaOH → CH₃COO⁻Na⁺ + NH₃

how do you reduce a carboxylic acid and give an example using ethanoic acid (word equation)

• dry lithium aluminium hydride (LiAlH₄), aka lithal or dry sodium borohydride (NaBH₄)

• in dry ether

• followed by dilute HCl(aq) to protonate the intermediate formed (acidic work up)

• reduces to an alcohol + water

• ethanoic acid + 4[H] → ethanol + water

how can you produce an acyl chloride? state conditions and give an example balanced equation starting from ethanoic acid including state symbols + observations. what type of reaction is this?

• by reacting a carboxylic acid with anhydrous phosphorous (V) chloride at room temperature (298K) as reaction is very vigorous

• CH₃COOH(l) + PCl₅(s) → CH₃COOCl(l) + POCl₃(l) + HCl(g)

• observations: effervesence and steamy fumes (HCl)

• substitution reaction

how do you form an ester? what type of reaction is the forward reaction? what type of reaction is the reverse reaction? give an example showing the balanced equation and word equation. what should be noted about this reaction?

• react a carboxylic acid with an alcohol in the presence of a concentrated sulphuric acid catalyst (+ heat)

• forward reaction - esterification

• reverse reaction - ester (acid) hydrolysis

• CH₃OH + CH₃COOH ⇌ CH₃COOCH₃ + H₂O

• methanol + ethanoic acid ⇌ methyl ethanoate + water

• this reaction is reversible and reaches a dynamic equilibrium

what is acylation?

replacing a hydrogen with an acyl group

• acyl chloride + water → ?

• e.g. ethanoyl chloride + water → ?

• what type of reaction is this?

• acyl chloride + water → carboxylic acid + HCl

• ethanoyl chloride + water → ethanoic acid + HCl

• substitution reaction

• acyl chloride + alcohol → ?

• e.g. ethanoyl chloride + methanol → ?

• what type of reaction is this and what are the conditions?

• acyl chloride + alcohol → ester + HCl

• ethanoyl chloride + methanol → methyl ethanoate + HCl

• esterification reaction

• this reaction is irreversible and goes to completion

• reaction does not require catalyst and occurs at room temperature (as acyl chlorides are highly reactive electrophiles)

• acyl chloride + concentrated ammonia → ?

• e.g. ethanoyl chloride + concentrated ammonia → ?

• acyl chloride + concentrated ammonia → an amide + ammonium chloride

• ethanoyl chloride + concentrated ammonia → ethanamide + ammonium chloride

• ethanoyl chloride + R-NH₂ (amine) → ?

• example equation using

• N-Rylethanamide + R-N⁺H₃ + Cl⁻

write a general word equation for the acid hydrolysis of an ester (reverse of esterification). then write a balanced equation for the acid hydrolysis of ethyl ethanoate and state the required conditions and reactants. what must be noted about this reaction?

• ester + water → carboxylic acid + alcohol

• reactant: H₂O

• catalyst: concentrated sulphuric acid, H₂SO₄

• CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH

• reaction is reversible and reaches a dynamic equilibrium

write a general word equation for the alkaline hydrolysis of an ester. then write a word equation and balanced equation for the alkaline hydrolysis of methyl ethanoate and state the required conditions and reactants. what must be noted about this reaction?

• ester + base (NaOH(aq)) → carboxylate salt + alcohol

• methyl ethanoate + sodium hydroxide → sodium ethanoate + methanol

• CH₃COOCH₃ + NaOH → CH₃CO^(-)Na⁽⁺⁾ + CH₃OH

• reactant: concentrated NaOH(aq)

• reaction is irreversible and goes to completion

how are condensation poylmers formed?

condensation polymers are formed when a water molecule is removed from the species of a reaction (so the reaction produces water therefore condensation reaction)

how is a polyamide formed?

when a dicarboxylic acid reacts with a diamine (also forms water).

how is a polyester formed? what type of reaction is this?

• when a dicarboxylic acid reacts with a diol (also forms water)

• condensation polymerisation reaction

what do reductions of aldehydes and ketones form?

• alcohols

• opposite of oxidation

give the steps to show how 2,4-DNPH could be used to distinguish between propanone and propanal.

• formation of yellow precipitate

• filter (each precipitate) then recrystallise products

• determination of melting points

• compare melting points to data book values

what is an acyl group + examples?

examples include acyl chlorides, esters, ketones, aldehydes and amides

under what conditions do reactions with acyl chlorides take place?

room temperature - 298K

describe how a crystalline solid is separated and purified

• filter/decant

• dissolve in minimum volume

• of hot solvent

• cool/leave (to crystallise) AND filter (under reduced pressure)

• wash with cold solvent/water AND dry (with method)

draw the transition state (bonds partially broken + partially formed) of 2-bromobutane undergoing a nucleophilic substitution reaction with an OH⁻ nucleophile. draw bond angles and state the type of mechanism this is.

S_N2

draw the transition state (bonds partially broken + partially formed) of this molecule undergoing a nucleophilic substitution reaction with an OH⁻ nucleophile. state the type of mechanism this is.

S_N1

how do you produce a carboxylic acid from an acyl chloride?

react the acyl chloride with water at room temperature

how do you convert a carboxylic acid into a carboxylate salt? what type of reaction is this? give an example balanced equation using ethanoic acid.

• reactant: dilute alkali, e.g. NaOH

• acid-base reaction

• CH₃COOH + NaOH → CH₃COONa + H₂O

OR

• reactant: metal, e.g. Mg

• redox reaction

• 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂

how do you convert a carboxylate salt into a carboxylic acid? what type of reaction is this? give an example balanced equation using sodium ethanoate.

• reactant: dilute acid e.g. HCl

• acid-base reaction

• CH₃COONa + HCl → CH₃COOH + NaCl

how do you convert an alcohol/ketone (containing CH₃CH(OH)- or CH₃CO- respectively)  carboxylic acid (with 1 fewer C atom in chain)

• reactants: iodine and aqueous sodium hydroxide (alkali)

• positive test → yellow precipitate (CHI₃ - triiodomethane, also known as iodoform) + antiseptic smell

• followed by dilute acidic work up to protonate the carboxylate salt to carboxylic acid

how do you convert an acyl chloride into an amide (primary, secondary and tertiary)? name the type of mechanism and reaction, and the reactants required. give example balanced equations starting from ethanoyl chloride to form ethanamide, N-ethylethanamide and N,N-diethylethanamide

• reactants: ammonia or an amine

• nucleophilic (acyl) substitution reaction

• CH₃COCl + 2NH₃  → CH₃CONH₂ + NH₄Cl (primary amide)

• CH₃COCl + 2CH₃CH₂NH₂ → CH₃CONHCH₂CH₃ + CH₃NH₂NH₃Cl (secondary amide)

• CH₃COCl + 2(CH₃)­₂NH → CH₃CON(CH₃)₂ + (CH₃)₂NH₂Cl (tertiary amide)                    

what reducing agent and conditions can be used to reduce a carboxylic acid? what type of reaction is this? what is formed?

• dry lithium aluminium hydride (LiAlH₄), aka lithal or dry sodium borohydride (NaBH₄)

• in dry ether

• followed by dilute HCl(aq) to protonate the intermediate formed (acidic work up)

• if a carboxylic acid is reacting then this is just a reduction

• an alcohol is formed

how do you convert an aldehyde/ketone into a hydroxynitrile?

• react the aldehyde/ketone with HCN and KCN catalyst

• KCN acts as a homogenous catalyst

what are the hazards associated with HCl gas?

HCl gas is toxic and corrosive

how do you convert an ester into a carboxylic acid?

acid hydrolysis

how do you convert an ester into a carboxylate salt?

alkaline hydrolysis

how do you convert a halogenoalkane to a Grignard? what type of reagent is a Grignard? give an example equation starting from 1-bromopropane.

• reactant: magnesium metal

• conditions: in dry ether

• the Grignard reagent is a nucleophile - it donates electrons from the C—Mg σ bond

what are the conditions for reactions with Grignards

• react with Grignard reagent in dry ether

• followed by a dilute acidic work up with HCl(aq)

how do you convert a Grignard into an alcohol (with increased carbon chain length - possible for primary, secondary and tertiary)? what type of mechanism and reaction is this? what are the conditions? give an example balanced equation(s) starting with CH₃CH₂MgBr being converted to CH₃CH₂CH₂OH.

• reactants: carbonyl compound (aldehyde or ketone), e.g. ethanal, followed by dilute acidic work up with HCl(aq)

• nucleophilic addition reaction

• CH₃CH₂MgBr + HCHO (methanal) → CH₃CH₂CH₂OMgBr 
  CH₃CH₂CH₂OMgBr + HCl(aq) → CH₃CH₂CH₂OH + MgBrCl

• a primary alcohol is formed with an extended carbon chain
  
  

write a balanced equation for the reaction between ethane-1,2-diol and ethanedioic acid using skeletal formula and then structural formula

n CH₂OHCH₂OH + n HOOCCOOH → n[OCH₂CH₂OCOCO] + 2n H₂O