Alkanes, alkenes &alkynes

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Organic Chemistry


<ul><li> ALKANES, ALKENES, ALKYNES NOMENCLATURE, STRUCTURES AND ISOMERISM </li> <li> Alkanes = CnH2n+2 Alkenes = CnH2n Alkynes = CnH2n-2 </li> <li> ALKANES, ALKENES, ALKYNES AND CYCLOALKANES ARE HYDROCARBONS (COMPOUNDS CONTAINING ONLY CARBON AND HYDROGEN). EACH OF THESE FORM A HOMOLOGOUS SERIES (A GROUP OF ORGANIC COMPOUNDS HAVING A COMMON GENERAL FORMULA/ OR IN WHICH EACH MEMBER FIFFERS FROM THE NEXT BY A CH2) </li> <li> THE HYDROCARBONS MAY BE SATURATED (CONTAINS ONLY SINGLE BONDS BETWEEN CARBON-CARBON ATOMS/ CARBON ATOMS BONDED TO THE MAXIMUM NUMBER OF HYDROGENS) OR UNSATURATED (CONTAINS AT LEAST A DOUBLE BOND BETWEEN C-C ATOMS) </li> <li> ALKANES: NOMENCLATURE Also called paraffins. A group of saturated hydrocarbons with the general formula Cn H2n+2 . They form a homologous series. Straight chain alkanes have their carbon atoms bonded together to give a single chain Alkanes may also be branched. </li> <li> NAMING (GENERAL) Hydrocarbon names are based on: 1) type, 2) # of carbons, 3) side chain type and position 1) name will end in -ane, -ene, or -yne 2) the number of carbons is given by a prefix 1 meth- 2 eth- 3 prop- 4 but- 5 pent- 6 hex- 7 hept- 8 oct- 9 non- 10 dec- Actually, all end in a, but a is dropped when next to a vowel. E.g. a 6 C alkene is hexene </li> <li> Determine the longest continuous chain (not always straight) in the molecule. The base name of the hydrocarbon is the name of the longest chain. </li> <li> IUPAC system </li> <li> IUPAC SYSTEM Name any chain branching off the longest chain as an alkyl group (e.g., methyl, ethyl etc) The complete name of a branch requires a number that locates the branch on the longest chain. Therefore number the chain in whichever direction gives the smaller number for all branches. </li> <li> 6. When two or more branches are identical, use prefixes (di-, tri-, etc.) (e.g. 2,4- dimethylhexane). Numbers are separated with commas. Prefixes are ignored when determining alphabetical order. (e.g. 2,3,5- trimethyl-4-propylheptane) 7. When identical groups are on the same carbon, repeat the number of this carbon in the name. (e.g. 2,2-dimethylhexane) </li> <li> Where there are two or more different alkyl branches, the name of each branch, with its position number precedes the name. the branch names are placed in alphabetical order. </li> <li> Alkenes and alkynes Both groups are unsaturated hydrocarbons. Each group is a homologous series. The main chain is defined as the chain containing the greatest number of double/tripple bonds We number the position of the double/tripple bond so that it has the lowest numbers. </li> <li> ALKENES </li> <li> alkynes </li> <li> Naming side chains Example: name the following structure CH3 CH2 C CH2 CH2 C CH2 CH3 CH3 CH3 Step 1 Identify the correct functional group </li> <li> Naming side chains CH3 CH2 C CH2 CH2 C CH2 CH3 CH3 CH3 Step 2 - find the longest chain </li> <li> Naming side chains CH3 CH2 C CH2 CH2 C CH2 CH3 CH3 CH3 Step 3 - add the prefix naming the longest chain </li> <li> Naming side chains CH3 CH2 C CH2 CH2 C CH2 CH3 CH3 CH3 Step 4 - number the longest chain with the lowest number closest to the double bond </li> <li> CH3 CH2 C 2 CH2 1 CH2 3 C 4 CH2 5 CH3 CH3 CH3 6 CH3 CH2 C CH2 CH2 C CH2 CH3 CH3 CH3 Naming side chains Step 5 - add that number to the name </li> <li> CH3 CH2 C 2 CH2 1 CH2 3 C 4 CH2 5 CH3 CH3 CH3 6 CH3 CH2 C CH2 CH2 C CH2 CH3 CH3 CH3 Naming side chains ethyl methyl methyl Step 6 - Name the side chains </li> <li> CH3 CH2 C CH2 CH2 C CH2 CH3 CH3 CH3 CH3 CH2 C 2 CH2 1 CH2 3 C 4 CH2 5 CH3 CH3 CH3 6 Naming side chains ethyl methyl methyl Step 7 - Place the side chains in alphabetical order &amp; name the compound </li> <li> name the following CH3 CH2 CH CH3 CH2CH2 CH3 CH3 CH CH CH3 CH CH3 CH2 CH2 CH3 CH2 CH3 CH3CH2CH CH CH CH2CH CH3 CH3 CH2CH3 CH3 CH3 </li> <li> CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH3 CH3 CH CH2 CH2 CH CH2 CH2 CH3 CH3 CH3 CH2 CH CH2 CHCH2 CH2 CH3 CH2 CH3 CH2 CH CH C CH3CH3 CH3 CH3 1 2 3 4 </li> <li> CH3 CH2 CH CH3 CH2CH2 CH3 CH3 CH CH CH3 CH CH3 CH2 CH2 CH3 CH2 CH3 CH3CH2CH CH CH CH2CH CH3 CH3 CH2CH3 CH3 CH3 9 1 0 1 1 </li> <li> CH3 C CH CH CH3 CH2 CH2 CH3 CH3 CH CH CH2 CH CH3 CH3 CH3 CH2 C C CH2 CH3 CH3 </li> <li> ISOMERS A GOOD TIME TO INTRODUCE ISOMERS (COMPOUNDS WITH THE SAME MOLECULAR FORMULA BUT DIFFERENT STRUCTURAL FORMULAE) TRY THE FOLLOWING: </li> <li> Reactions of alkanes &amp; alkenes We study three particular reaction cases: Substitution Addition Elimination Combustion </li> <li> Reactions of alkanes Substitution (of H, commonly by Cl or Br) Combustion (conversion to CO2 &amp; H2O) </li> <li> Combustion When alkanes are heated in a plentiful supply of air, combustion occurs Alkanes are energetically unstable with respect to water and carbon dioxide They only burn when they are in the gaseous state Explain what happens when a candle burns! </li> <li> 2 C4H10(g) + 13 O2(g) 8 CO2(g) + 10 H2O(g) 2 C8H18(l) + 25 O2(g) 16 CO2(g) + 18 H2O(g) </li> <li> SUBSTITUTION Reactions with chlorine Alkanes only react with chlorine when a mixture of the two is exposed to sunlight or ultraviolet light The light provides the energy required to break the very strong bonds This is an example of a substitution reaction </li> <li> In the presence of light, or at high temperatures, alkanes react with halogens to form alkyl halides. Reaction with chlorine gives an alkyl chloride. CH4(g) + Cl2(g) CH3Cl(g) + HCl(g) </li> <li> Cracking Cracking happens when alkanes are heated in the absence of air The products of the cracking of long-chain hydrocarbons are shorter chain molecules Ethane is cracked industrially to produce ethene </li> <li> PHYSICAL PROPERTIES Alkanes are non polar so they are insoluble in water but soluble in each other. Low molecular alkanes are gases. Boiling points increase with increasing chain length (molecular weight) for the first few members Boiling points decrease with increasing number of branches.(Explain this in terms of Van der Waals forces and surface area. </li> <li> Melting and boiling points increase with increased molecular weight (Methane bp. -164C, decane bp. 174C) While boiling point decrease with chain branching (decrease in surface area), melting points increase Alkanes are less dense than water and swim on top of water </li> <li> alkenes: preparation and reactions </li> <li> Alkenes: Preparation and reactions Two ways of making alkenes: 1. Heat a concentrated solution of potasium /sodium hydroxide in alcohol (alcoholic KOH) with a haloalkane (halogenoalkane) This is dehydrohalogenation (removal of hydrogen and halogen) 2. Heat concentrated sulphuric acid with the alcohol- dehydration. THE ACID IS A DEHYDRATING AGENT </li> <li> i) Dehydration of alcohols conc. H2SO4 R-CH2-CH2- OH R-CH=CH2 + H2O ii) Dehydrohalogenation of haloalkanes NaOH/ethan olR-CH2-CH2-X reflux R-CH=CH2 + HX NaOH can be replaced by KOH </li> <li> LEARNERS MUST KNOW MAJOR PRODUCTS IN ALL CASES AND REACTION CONDITIONS CH3CH2-CH-CH3 OH H+ H+ CH3CH=CH-CH3 + H2O CH3CH2-CH=CH2 + H2O 2-butanol 2-butene major product 1-butene Dehydration of alcohols Dehydrohalogenation of haloalkanes CH3CH-CH-CH2 BrH H KOH CH3CH=CH-CH3 CH3CH2CH=CH2 alcohol reflux 2-bromobutane 2-butene (major product) 1-butene </li> <li> REACTIONS OF ALKENES (VIP) </li> <li> Catalytic hydrogenation: - hydrogenation: addition of hydrogen to a double bond and triple bond to yield saturated product. - alkenes will combine with hydrogen in the present to catalyst to form alkanes. C C H H C C H H Pt or Pd 25-90o C - Plantinum (Pt) and palladium (Pd) Catalysts - Pt and Pd: temperature 25-90oC - Nickel can also used as a catalyst, but a higher temperature of 140oC 200oC is needed. </li> <li> Addition of halogens: i) In inert solvent: - alkenes react with halogens at room temperature and in dark. - the halogens is usually dissolved in an inert solvent such as dichloromethane (CH2Cl2) and tetrachloromethane (CCl4). - Iodine will not react with alkenes because it is less reactive than chlorine and bromine. - Fluorine is very reactive. The reaction will produce explosion.C C X X C C X X inert solvent X X = halogen such as Br2 or Cl2 Inert solvent = CCl4 or CH2Cl2 </li> <li> EXAMPLES: C C HH H H Br Br Br2 Br Br CCl4 CH3CH=CH2 Cl2 CCl4 CH3CH Cl CH2 Cl C C Br H H Br H H inert solvent (CCl4) ethene 1,2-dibromoethane * the red-brown colour of the bromine solution will fade and the solution becomes colourless. cyclohexene 1,2-dibromocyclohexane propene 1,2-dichloropropane </li> <li> Hydrohalogenation </li> <li> MARKOVNIKOVS RULE ( A statement of the rule is not needed) There are 2 possible products when hydrogen halides react with an unsymmetrical alkene. It is because hydrogen halide molecule can add to the C=C bond in two different ways. C C H HCH3 H H-I C C H HCH3 H H-I C C H HCH3 H H I C C H HCH3 H I H 1-iodopropane 2-iodopropane (major product) </li> <li> Markovnikovs rules (Not for examination) - the addition of HX to an unsymmetrical alkene, the hydrogen atom attaches itself to the carbon atom (of the double bond) with the larger number of hydrogen atoms. </li> <li> Addition reaction with concentrated sulfuric acid: hydration of alkenes - the alkene is absorbed slowly when it passed through concentrated sulfuric acid in the cold (0-15oC </li> <li> Addition reaction with acidified water (H3O+): hydration of alkenes Hydration: The addition of H atoms and OH groups from water molecules to a multiple bond. Reverse of the dehydration reaction. Direct hydration of ethene: - passing a mixture of ethene and steam over phosphoric (v) acid (H3PO4) absorbed on silica pellets at 300oC and a pressure of 60 atmospheres. - H3PO4 is a catalyst. </li> <li> CH2=CH2 H2O H3PO4 CH3CH2OH(g) (g) 300 o C, 60 atm (g) ethene ethanol C C H2O C C H OH alkene alcohol </li> <li> Oxidation (Combustion of alkenes) The alkenes are highly flammable and burn readily in air, forming carbon dioxide and water. For example, ethene burns as follows : C2H4 + 3O2 2CO2 + 2H2O </li> <li> HALOGENOALKANES Halogenoalkanes are compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms (fluorine, chlorine, bromine or iodine). </li> <li> Functional group = halogen Ex. Fluorine = fluoro Number by which carbon attached to, put in alphabetical order Ex. Bromoethane </li> <li> Halogenoalkanes fall into different classes depending on how the halogen atom is positioned on the chain of carbon atoms. There are some chemical differences between the various types. Primary Secondary Tertiary </li> <li> Primary (1) carbon carrying halogen is attached to only one carbon alkyl group Secondary (2) carbon carrying halogen is attached to two other alkyl groups Tertiary (3) carbon carrying halogen is attached to three alkyl groups </li> <li> Reactions of the halogenoalkanes Substitution:In a substitution reaction, one atom or group of atoms, takes the place of another in a molecule. Elimination: Halogenoalkanes also undergo elimination reactions in the presence of sodium or potassium hydroxide which is dissolved in ethanol. </li> <li> Example of substitution When an aqueous solution of NaOH or KOH is added to haloalkane an alcohol is produced. propan-2-ol </li> <li> Example of elimination what conditions are needed? </li> <li> ALCOHOLS (CnH2n+1OH) Preparation and properties </li> <li> nomenclature Select the longest chain which contains the OH group and number so that the OH group has the smallest number. See the examples below </li> <li> Classification In a primary (1) alcohol, the carbon which carries the -OH group is only attached to one alkyl group. In a secondary (2) alcohol, the carbon with the -OH group attached is joined directly to two alkyl groups, which may be the same or different. </li> <li> In a tertiary (3) alcohol, the carbon atom holding the -OH group is attached directly to three alkyl groups, which may be...</li></ul>


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