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Medicines by Design Revision of reagents, conditions and reaction types: Aliphatic chemistry reagents and conditions; Aliphatic chemistry reaction types; Aromatic chemistry reagents and conditions; Aromatic chemistry reaction types; Key words and definitions; A10. Aliphatic reactions: add the reagents and conditions R H R C C H O 9 2 CH2 O 9 C R OH R O C R H O R' 8 X10 4 SCl2O Heat + reflux 1 5 7 O R CH2 1 R Br CH2 CN R 4 6 O R R CH2 NH2 R C NH2 CH3 O R Cl Dil H2SO4 Heat + reflux 3 6 C NaCN in ethanol/H2O Heat + reflux R C OH 9 H 7 CH2 COOH C NH R' Print a hard copy of this page; Add reagents and conditions to the arrows; Using the hyperlinks will direct you to the relevant notes; Click here to check your answers once you have finished. A12. Aliphatic reagents and conditions X12 O R O H C R C H CH2 O C R OH R C H R OH H C O O R CH2 R Br CH2 CN R R' O C R Cl C NH2 O R R CH3 R CH2 NH2 R CH2 COOH C NH R' Print some hard copies of this page to practice reagents and conditions A11. Aliphatic reagents and conditions X11 H C Al2O3 300oC R C H CH2 H O O O C R OH R’OH/conc. H2SO4 cat heat + reflux H C R OH Conc. HBr room temp SCl2O heat + reflux NaOH(aq) heat + reflux NaBr(s) + conc. H2SO4 heat + reflux NaBH4 room temp Dil. HCl or H2SO4 or NaOH Heat + reflux R H2/Ni cat 150oC + 5 atm CH2 R Br CH2 CN R C R’NH2 room temp Conc. NH3 heat + reflux Dil. H2SO4 heat + reflux R CH3 R CH2 NH2 R C NH2 O R R' O R Cl NaCN in ethanol/H2O heat + reflux Br2(l) sunlight O Conc. NH3 room temp O R C R’OH room temp R K2Cr2O7(aq)/dil. H2SO4 heat + reflux K2Cr2O7(aq)/dil. H2SO4 heat + reflux CH2 COOH C NH R' Print a hard copy of this page Learn the reagents and conditions Click here to go to a blank copy of this page to practice with A13. Aliphatic reactions: add the reaction types X13 O R O H C R C H CH2 O C R OH R C H R OH H C O H2/Ni cat 150oC + 5 atm O R CH2 R Br CH2 CN R O C R Cl O R CH3 R CH2 NH2 R CH2 COOH C NH C NH2 R R' R' Print a hard copy of this page Add the reaction types to the arrows Click here to go to the Key words and definitions for help Click here to check your answers once you have finished. A14. Aliphatic reaction types Esterification or condensation X14 R H C Elimination or dehydration R C H CH2 Oxidation O O C R OH R C H R OH Reduction H Addition or R Br CH2 CN R Nucleophilic substitution or acylation C NH2 O R R CH3 R CH2 NH2 R C Nucleophilic Substitution or acylation Hydrolysis Radical substitution R' O R Cl Nucleophilic substitution Nucleophilic substitution hydrogenation CH2 O Nucleophilic substitution or acylation O R C Hydrolysis Nucleophilic substitution Nucleophilic substitution Electrophilic addition O Oxidation CH2 COOH C NH R' Print a hard copy of this page Learn the reagents and conditions Click here to go back to a blank page that you can print off a few times to help you practice A1. The addition reactions of alkenes with the following: bromine, hydrogen in the presence of a catalyst, hydrogen bromide and water in the presence of a catalyst. CI 12.2 Ethene is unsaturated; it has a double C=C bond. It undergoes electrophilic addition reactions in which another molecule is added on. A saturated compound is produced. a. Ethene with bromine (g) or (l) or dissolved in an organic solvent. Ethene will decolourise orange bromine; it reacts rapidly to form a colourless compound. This occurs at room temperature: H H H2C CH2(g) + Br Br(g) H C C H(l) Br Br 1, 2-dibromoethane (colourless) R C (reagent) is Br2(g) or Br2(l) or Br2 in an organic solvent (N.B. Salters includes catalysts in this category if required) (conditions) are room temperature (N.B. Salters includes pressures here if required) b. Ethene with bromine water Br2(aq) H H H2C CH2 (g) + Br2 (aq) + H2O(l) H C C H (l) + HBr(aq) Br OH (orange) a bromoalcohol (colourless) Bromine water (bromine dissolved in water) is more convenient and safer to use as a test of unsaturation, than bromine (g) or bromine (l) or bromine dissolved in an organic solvent such as cyclohexane. R C (reagent) is Br2(aq) (conditions) are room temperature Back to map c. Ethene with concentrated hydrobromic acid, HBr(aq) CH2 CH2 + HBr CH3 CH2Br bromoethane (now saturated) R conc. HBr(aq) C room temperature d. Ethene with hydrogen(g) CH2 Hydrogenation CH2 + H2 CH3 CH3 ethane R H2(g) and Ni (or Pt) catalyst, finely powdered C 150°C and 5 atm with the Ni (or room temp. and pressure with the Pt) N.B. Pt is white gold: very expensive! e. Ethene with water H2O(g) (it is steam at 300°C) The catalyst is phosphoric acid absorbed onto silica (I imagine silica as being like pure white sand and that it is soaked in the acid which is a liquid). CH2 CH2 + H2O CH3 CH2OH ethanol R C H2O(g) cat. phosphoric acid absorbed onto silica 300°C and 60 atmospheres pressure Note: Reactions a, b, c, d and e are all examples of addition reactions. Reactions a, b, c and e are examples of electrophilic addition reactions. Reaction d involves a catalyst and has a different mechanism. Back to map The mechanism of the electrophilic addition reaction between bromine and alkenes curly arrows CI 12.2 show movement of a pair of electrons Mechanism for reaction in (a) above: H H H C = C H H H C + C H + - Br : H Br a carbocation and a bromide ion + Br δ Br δ The bromine molecule is polarised i.e it has a δ+ and a δ- atom like when the bond is polar. This happens because the electrons in the Br 2 molecule are repelled by the electrons in the C=C. The Brδ+ end now acts as an electrophile. Electrophiles are attracted to a negatively charged region (the C=C is made up of 4 electrons) and go on to accept a pair of electrons. N.B. ‘philic’ means ‘to like’ and ‘phobic’ means ‘to hate’. The carbocation has a C with a positive charge; that C had a half share of the two electrons in the bond which was broken. It now has no share of that pair and is now a C atom with one less electron so it has a single positive charge. The bromide ion is written: One of the lone pairs has come from the bond in the Br 2 molecule. It had a share of the bonding pair but now has both electrons. It now has overall gained an electron so the Br atom now has a negative charge. Br Then we have the next step: H H + C C H H H H C C Br Br H H Br - Br : 1, 2 dibromoethane This is electrophilic addition. It is very important that you understand this mechanism and are able to explain it. The other reactions of ethene proceed by a similar mechanism. Back to map A2. The dehydration of alcohols to form alkenes CI 13.4 In a dehydration reaction, a molecule of water is lost. Sometimes this is called an elimination reaction. saturated unsaturated This is the reverse of an addition reaction: e.g. CH3CH2OH CH2=CH2 + H2O What are the alternative reagents and conditions for this reaction? Reagents: ethanol passed over alumina (Al2O3) catalyst; Conditions: 300oC; N.B. phenols and carboxylic acids do not undergo dehydration reactions. The meaning of the term elimination reaction CI 13.4 In an elimination reaction, a small molecule e.g. water, is lost. Dehydration of alcohols are elimination reactions. Write two equations for the elimination of water from propan-1-ol. In the first, use full structural formulae; in the second, use skeletal formulae. Back to map A3. The reaction of alkanes with halogens CI 6.3 Examples: CH4 + C6H14 + Cl2 CH3Cl + Br2 C6H13Br + HCl equation (1) HBr These are both radical substitution reactions where the halogen replaces hydrogen. This type of reaction leads to a mixture of halogen substituted products because we cannot control the radicals that are produced. Mixture of products The reaction of methane (CH4) with chlorine in the presence of sunlight proceeds via a radical chain reaction. The overall equation for the reaction is equation (1) above. Complete the reaction mechanism below: Initiation: Cl2 2Cl. Propagation: CH4 + Cl. CH3. + HCl CH3. + Cl2 CH3Cl + Cl. (chloromethane made) CH3Cl + Cl. CH2Cl. + HCl CH2Cl. + Cl2 CH2Cl2 + Cl. (dichloromethane made) Termination: Back to map A4. Nucleophilic substitution reactions of the halogenoalkanes with hydroxide ions and ammonia CI 13.1, Act A4.1b The feature of a halogenoalkane molecule that allows it to undergo substitution reaction is the presence of a polar bond between the halogen atom and the carbon atom to which it is bonded. The halogen atom is slightly negatively charged and the carbon atom is slightly positively charged. This carbon atom is attacked by nucleophiles. + C - Br Why is the carbon – halogen bond polar? Complete this table for two common nucleophiles: Formula OH - Structure [inc. lone pairs] _ .. :O:H .. Reaction Organic product R-X + OH- R-OH + X- NH3 alcohol amine The mechanism of nucleophilic substitution in halogenoalkanes CI 13.1 In general: where X – is any nucleophile and the curly arrow shows the movement of a pair of electrons. The C – Hal bond will break heterolytically! Back to map A5. An outline of the preparation of a halogenoalkane from an alcohol Act A4.2 One way to make a halogenoalkane is to start with an alcohol and replace the –OH group by a halogen atom. Reaction in Activity A4.2: CH 3 CH 3 CH 3 C OH + CH H Cl 3 Cl + CH 3 CH 3 2- methylpropan-2-ol C 2-chloro-2-methylpropane This is an example of nucleophilic substitution – the alcohol group is first protonated in the strongly acid solution before it can be displaced by the Cl – nucleophile (water is lost). Give the mechanism for this reaction: (CI 13.1, p303 may help) R NaBr(s) and concentrated H2SO4 C heat and reflux Back to map H2O A6. Amines can act as nucleophiles with acyl chlorides CI 13.8 Ammonia/amines react with acyl chlorides: ammonia + acyl chloride primary amine + primary amide acyl chloride + secondary amide HX + HX These are nucleophilic substitution reactions. Complete the following equations: O + H N H Cl H C + CH3 an acylchloride ammonia primary amide O CH3 N H + Cl C + CH3 H methylamine (a primary amine) an acylchloride secondary amide These reactions are very vigorous even at room temperature. O + CH3 - C Cl The C has two more electronegative groups attached so they are very attractive to nucleophiles e.g. NH3, amines. - The reactions of amines and acyl chlorides are sometimes called acylation reactions. Back to map A7. Ester formation from alcohols or phenols CI 13.5 Esters are named after the alcohol and the acid from which they are made: Complete the following table: Acid Alcohol Resulting Ester methyl methanoate H H H C C O H C OH C OH H H propanoic acid H C H O OH methanoic acid H methanol H H H H C C C OH H H H propan-1-ol Back to map Examples: acid + alcohol ester H 3C + C H + H OH H 3C + C H H OH C C OH ethanoic acid CH3 H water H3C + C O H O H H H C C C H H CH2CH3 water ethyl ethanoate H H H O ethanol O O + C methyl ethanoate H H ethanoic acid c. H3C OH methanol O C C H ethanoic acid H 3C O O OH b. water H O a. + O OH H3C H propanol + C O CH2CH2CH3 O H H water propyl ethanoate N.B. all of these reactions in which esters are made are reversible reactions. The forward reaction is called esterification. The backward reaction is called hydrolysis. In Activity WM2, an ester was hydrolysed. The catalyst was sodium hydroxide solution. When making an ester using a phenol rather than an alcohol, a more vigorous reagent than a carboxylic acid is needed. The OH group in a phenol is less reactive than the OH group in an alcohol. Method 1 OH phenol + + O HO O C CH3 carboxylic acid O C CH3 ester + + water (conc. H2SO4 catalyst and heat under reflux is needed even with an alcohol) Back to map H2O Method 2 O OH O + O phenol + O C CH3 O C CH3 C CH3 acid anhydride ester + + HO O C CH3 carboxylic acid (This does not need heat and reflux. It is reactive but not too dangerous. This method was used to make aspirin in WM.) Method 3 OH phenol + + O Cl O O C CH3 C CH3 acyl chloride ester + + hydrochloric acid (Faster but ethanoyl chloride is toxic and hazardous as it is so very reactive) Why do you think both methods 2 and 3 must be carried out in the absence of water? Both acid anhydrides and acyl chlorides are called acylating agents: -OH on alcohol or on phenol is replaced by R C O in the ester. This is an acyl group. O Back to map HCl A8. The hydrolysis of esters CI 13.5 The reverse of esterification is hydrolysis. Hydrolysis is bond breaking involving water: O H 3C + C O O H H H C C H H H CH2CH3 ethyl ethanoate H water ethanol O OH + H3C C OH ethanoic acid On hydrolysis, the sweet smell of the ester disappears. To speed up this reaction NaOH (aq) is often used when heating under reflux. This reaction was used in the hydrolysis of Oil of Wintergreen in WM. What ion is produced instead of ethanoic acid if NaOH(aq) is used as a catalyst? Give name and full structural formula. What needs to be added now to produce ethanoic acid from the ethanoate ion? Back to map A9. The following reactions involving aldehydes and ketones: formation by oxidation of alcohols, oxidation to carboxylic acids and reduction to alcohols CI 13.7 Recognising aldehydes and ketones Aldehydes and ketones are both carbonyl compounds which have the carbonyl group: O C Aldehydes are made when a primary alcohol is oxidised. Examples: [2 C’s → eth; suffix -al] H O H C C 23 24 H H ethanal H H H O C C C [3 C’s → prop; suffix –al] H H H propanal In aldehydes the carbonyl group is always on the end of the C chain so no number is required in the name. Ketones are made when a secondary alcohol is oxidised. Examples: O H3C C 41 CH3 propanone H H C 44 H C 45 H O C C 46 47 H H H pentan-2-one H C 48 H H The carbonyl group is midway along the carbon chain. A number may be required to show which C atom has the carbonyl group. Formation of aldehydes and ketones from alcohols: Remember a triangle! primary alcohol secondary alcohol oxidation aldehyde oxidation oxidation carboxylic acid aldehyde tertiary alcohol A good oxidising agent is acidified potassium dichromate (VI) solution. Back to map Here is the half equation showing what happens to the dichromate ions: Cr2O72- + 14H+ + 6e- 2Cr3+ + 7H2O Oxidation numbers: +6 (acid) → +3 Colour change: orange → green This is the half equation for the oxidation of ethanol: CH3-CH2-OH + H2O CH3CHO + 2H+ + 2e- Combine the 2 half equations above and write an overall equation for the redox reaction: Definitions of oxidation are: gain of oxygen; loss of hydrogen; loss of electrons; increase in oxidation number; Give four definitions of reduction: Is the chromium in the half equation above oxidised or reduced? Back to map A closer look at the reaction of the primary alcohol: H H H C C H H H H OH C C H H H a primary alcohol H O C O C OH H b aldehyde carboxylic acid Why are both of these steps described as oxidation? a. b. A closer look at the reaction of a secondary alcohol: H H H H C C C H OH H secondary alcohol H H H H C C C H O H ketone Why doesn’t oxidation of the ketone occur? H H H C C H O OH carboxylic acid A closer look at the reaction of a 3ry alcohol: H H C H H H H C C C H OH H H H H tertiary alcohol H C C C H O H H ketone Why doesn’t oxidation of a tertiary alcohol occur? N.B. phenols and carboxylic acids are not oxidised either! It is important that you can quote the reagents and conditions for these oxidations: Reagents – K2Cr2O7(aq), dilute HCl(aq) or dilute H2SO4(aq) Conditions – heat and reflux; If oxidation occurs, there will be a colour change from orange to green. Back to map Reduction of carbonyls to alcohols Aldehyde primary alcohol Ketone secondary alcohol R = NaBH4(aq) C = room temperature Name the reagent: Give the formulae of the carbonyl compounds which would be reduced to these alcohols: This would be reduced to the alcohol on the right . . . CH3 This would be reduced to the alcohol on the right . . . CH2 OH CH3 CH CH CH3 CH3 C CH3 Back to map CH3 CH2 OH Aromatic: add the reagents and conditions Cl Br R7 H2(g), Ni catalyst, 300°C, 30 atm. SO2OH 1 1 3. 2 NO2 Print a hard copy of this page Add reagents and conditions to the arrows Using the hyperlinks will direct you to the relevant Notes Click here to check your answers once you have finished Sn + conc. HCl; heat + reflux; NH2 6 4 5 + N N Cl- O R C 6 R N N OH Aromatic reagents and conditions Cl Br Print a hard copy of this page Learn the reagents and conditions Click here to go to a blank page to print off and practice with R8 Br2(l), FeBr3 cat. room temp. H2(g), Ni catalyst; 300°C, 30 atm. SO2OH conc. H2SO4. heat + reflux Cl2(g), AlCl3 cat. room temp/ anhydrous conc. HNO3, conc. H2SO4 cat. <55°C RCl(l), AlCl3 cat. heat + reflux/ anhydrous NO2 Sn + conc. HCl heat + reflux NH2 NaNO2(aq)/dil. HCl <5°C RCOCl(l), AlCl3 cat. heat + reflux/ anhydrous + N N Cl- O R C alkaline phenol <5°C R N N OH Aromatic reagents and conditions Cl Br Run off a few hard copies of this page. Use it to practice reagents and conditions. R9 NO2 SO2OH NH2 + N N Cl- O R C R N N OH Aromatic reaction types Cl Br C10 Run off a hard copy of this page Add the reaction types Click here to go to the key words for help Click here to check your answers once you have finished NO2 SO2OH NH2 + N N Cl- O R C R N N OH Aromatic reaction types Cl Br Run off a hard copy of this page Learn the reaction types Click here to go to a blank page you can print to practice with C11 Electrophilic substitution Hydrogenation SO2OH Electrophilic substitution Electrophilic Substitution or Friedel Crafts alkylation Electrophilic substitution Electrophilic substitution NO2 NH2 Reduction Diazotisation Electrophilic Substitution or Friedel Crafts acylation + N N Cl- O R C R Coupling reaction R1. Halogenation of the ring in arenes CI 12.4 Electrophilic substitution . . . The H+ reacts with the FeBr4-. Step 1: The bromine molecule is polarised as it approaches the benzene ring. + Br + Br - FeBr4- Br The FeBr3 helps to polarise the bromine by accepting a lone pair of electrons from one of the bromine atoms. FeBr3 The bromine molecule is so polarised that it splits. Step 2: Br Br+ FeBr4- H Why does the benzene attract electrophiles? R Br2(l) and either FeBr3 or Fe(s) C room temperature E C6H6 + Br2 C6H5Br + HBr Br FeBr3 Br+ becomes bonded to a ring C and H+ is lost. Br+ is an electrophile: has a +ve charge; attracted to electron rich centre; accepts pair of electrons to form a dative covalent bond; FeBr3 is regenerated as it is a catalyst in the reaction. What would the formula of the product have been if addition had occurred rather than substitution? Back to map Chlorine reacts in a similar way to bromine. Complete the mechanism: Electrophilic substitution . . . Step 1: The chlorine molecule is polarised as it approaches the benzene ring. + + Cl Br Br - ClBr + The AlCl3 helps to polarise the chlorine by accepting a lone pair of electrons from one of the chlorine atoms. FeBr4The chlorine molecule is so polarised that it splits. FeBr AlCl33 Step 2: Br Br+ FeBr4- Cl+ becomes bonded to a ring C and H+ is lost. Cl+ is an electrophile: has a +ve charge; attracted to electron rich centre; accepts pair of electrons to form a dative covalent bond; R C E H Br The H+ reacts with the AlCl4-. AlCl3 is regenerated as it is a catalyst in the reaction. Cl2(l) and AlCl3(s) room temperature, anhydrous C6H6 + Cl2 C6H5Cl + HCl Back to map FeBr3 R2. The nitration of benzene CI 12.4 Electrophilic substitution . . . R Conc. nitric acid and conc. sulphuric acid as a catalyst < 55°C; Explain why . . . C C6H6 + HNO3 C6H5NO2 + H2O E Step 1: HNO3 + 2H2SO4 NO2+ + 2HSO4- + H2O The nitrating mixture NO2+ is an electrophile Step 2: NO2 NO2+ + HSO4 - Why do we say that the sulphuric acid is a catalyst? Back to map + H2SO4 R3. The sulphonation of benzene CI 12.4 Electrophilic substitution . . . O OH S E + H2SO4 R Conc. sulphuric acid C Heat and reflux O + H2O Draw the structural formula of the electrophile and name it: Name the organic product of this reaction: Back to map R4. The Friedel-Crafts alkylation of benzene CI 12.4 Electrophilic substitution . . . CH3 E + CH3Cl + HCl Why Friedel-Crafts? Why alkylation? Give the reagents and conditions: R C How would you do this experiment differently if you wanted to make ethylbenzene rather than methylbenzene? Back to map R5. The Friedel-Crafts acylation of benzene CI 12.4 In these reactions, an acyl group is introduced into the benzene ring. O C CH3 Acyl chlorides also have acyl groups in. Draw the full structural formula of propanoyl chloride and also circle the acyl group: O O E R C + CH3 C C CH3 Cl Give the reagents: Give the conditions: Friedel-Craft reactions are so useful to chemists because they provide a way of adding carbon atoms to the benzene ring. Back to map + HCl R6. The formation of azo dyes by coupling reactions involving diazonium compounds CI 13.10 Coupling reactions: diazonium salt + coupling agent azo compound Diazonium salts: A typical diazonium salt is benzenediazonium chloride; + N Cl- N This is made from phenylamine. The reaction is called diazotization. + NH2 + HNO2 + HCl N N R = NaNO2(aq) and dilute HCl Name the reagents: Why does the temperature have to be less than 5°C? Azo compounds: This is an azo compound. Circle the azo group: R N N Back to map R' Cl- + 2H2O If R and R’ are aromatic then the azo compound is more stable than if they were aliphatic. This is because the azo compound then has an extended delocalised system of electrons. Aromatic azo compounds are coloured and used as dyes. The coupling agent reacts with the diazonium salt to make the azo compound. The coupling agent is another compound containing a benzene ring e.g. phenol or phenylamine. + N N Cl- + OH N N Name the two organic reagents above . . . R = the phenol is in an alkaline solution for this reaction; C = at a temperature less than 5°C the coloured azo compound is formed immediately. Is the diazonium salt an electrophile or a nucleophile here? What colour is the azo compound above? Write the equation for the formation of another azo compound: Key Words Back to map OH + HCl Acylation The introduction of an acyl group, RCO-, using an acyl chloride. Typical of alcohols, amines, ammonia and arenes. Addition The addition of atoms or groups of atoms across a double bond. Typical of alkenes, aldehydes and ketones. Alkylation The introduction of an alkyl group, R, using a chloroalkane. Typical of arenes. Carbocation An organic molecule containing a carbon atom with a +ve charge. Intermediates in the electrophilic addition reactions of alkenes. Condensation A reaction in which two molecules join together and a small molecule such as H2O or HCl is eliminated. An example is the formation of an ester. Coupling reaction The reaction between a diazonium ion and another aromatic compound to form an azo compound, R-N=N-R’ Used to make azo dyes. Dehydration A term sometimes used to describe the elimination of a water molecule from an alcohol to form an alkene. Diazotisation The formation of a diazonium salt from an aromatic amine. Electrophile positive ion or molecule with +; attracted to an electron rich centre; accepts a pair of electrons to make a dative covalent bond; Electrophilic addition Typical of alkenes. Electrophilic substitution Typical of arenes since delocalisation is retained. Elimination The loss of atoms or groups of atoms to produce an unsaturated compound. Typical of primary and secondary alcohols. Esterification A reaction in which an ester is formed from an alcohol and a carboxylic acid. Friedel Crafts acylation The introduction of an acyl group, RCO-, into a benzene ring. Named after its discoverers. Friedel Crafts alkylation The introduction of an alkyl group into a benzene ring. Named after its discoverers. Hydrogenation The addition of hydrogen atoms across a C=C. Hydrolysis Nucleophile A bond breaking reaction involving water often catalysed by dilute acid or alkali. Typical of esters, amides and nitriles (R-CN). a negative ion or a molecule with a lone pair of electrons; attracted to a positive/electron deficient centre; donates a pair of electrons to form a dative covalent bond; Nucleophilic addition Typical of aldehydes and ketones. Nucleophilic substitution Typical of halogenoalkanes. Oxidation Radical Radical substitution Reduction gain of oxygen; loss of electrons; loss of hydrogen; increase in oxidation number; Typical of primary alcohols, secondary alcohols and aldehydes. Atom or molecule with an unpaired electron. These are very reactive. Typical of alkanes. loss of oxygen; gain of electrons; gain of hydrogen; decrease in oxidation number; Typical of alkenes, aldehydes and ketones.