Hydrocarbon is a compound consisting of hydrogen and carbon only Molecular formula: The formula which shows the actual number of each type of atom Saturated: Contain single carbon-carbon bonds only Unsaturated : Contains a C=C double bond General formula: algebraic formula for a homologous series e.g. CnH2n Empirical formula: shows the simplest whole number ratio of atoms of each element in the compound 3.1 Organic: Basic Concepts Basic definitions to know Displayed formula: show all the covalent bonds and atoms present in a molecule When drawing organic compounds add the hydrogen atoms so that H each carbon has 4 bonds C C C H H H H H H C C H H H H H Drawing Displayed formulae Remember that the shape around the carbon atom in saturated hydrocarbons is tetrahedral and the bond angle is 109.5o C C H H H H H H N Goalby chemrevise.org 1 Skeletal formula shows the simplified organic formula, shown by removing hydrogen atoms from alkyl chains, leaving just a carbon skeleton and associated functional Groups. Structural formula shows the minimal detail that shows the arrangement of atoms in a molecule, eg for butane: CH3CH2CH2CH3 or CH3 (CH2 )2CH3
Organic compounds can be represented by:
• empirical formula
• molecular formula
• general formula
• structural formula
• displayed formula
• skeletal formula.
Students should be able to:
• draw structural, displayed and skeletal formulas for given organic compounds
Homologous series are families of organic compounds with the same functional group and same general formula. •They show a gradual change in physical properties (e.g. boiling point). • Each member differs by CH2 from the last. • same chemical properties. Functional group is an atom or group of atoms which when present in different molecules causes them to have similar chemical properties homologous series functional group prefix / suffix (* = usual use) example Alkane -ane CH3CH2CH2CH3 Butane Alkenes suffix -ene Alcohols suffix* -ol prefix hydroxyHalogenoalkanes prefix chloro- bromoiodo- Aldehydes suffix -al prefix formylKetones suffix* -one prefix oxocarboxylic acids suffix -oic acid Esters -yl –oate C C C OH H H C H C H H C H H O H C halogen H C C H Cl H H H C H H C O H C C O H H H H C O C C O C H H H H H H C O OH C C O OH H H H C C O O H H H C H H H propene Propan-1-ol 1-chloropropane ethanal Propanone Ethanoic acid C methylethanoate O O C C C C H C H H H H H OH Cl O O OH O O O code no of carbons meth 1 eth 2 prop 3 but 4 pent 5 hex 6 hept 7 oct 8 non 9 dec 10 General rules for naming carbon chains Count the longest carbon chain and name appropriately Find any branched chains and count how many carbons they contain Add the appropriate prefix for each branch chain Eg -CH3 methyl or -C2H5 ethyl –C3H7 propyl 3,5-dimethylheptane Basic rules for naming functional groups N Goalby chemrevise.org 3 H3C CH CH2 CH2 CH3 CH CH2 CH3 CH3 1 2 3 4 5 6 7 The position of the functional group on the carbon chain is given by a number – counting from the end of the molecule that gives the functional group the lowest number. For aldehydes, carboxylic acids & nitriles, the functional group is always on carbon 1. Butan-1-ol We only include numbers, however, if they are needed to avoid ambiguity. H C C C H H H H H H C H H H methylpropane Words are separated by numbers with dashes numbers are separated by commas If there is more than one functional group or side chain, the groups are listed in alphabetical order (ignoring any di, tri). The suffix for alkenes can go in front of other suffixes. •When using a suffix, add in the following way : If the suffix starts with a vowel- remove the –e from the stem alkane name e.g. Propan-1-ol, butan-1-amine, ethanoic acid, ethanoylchloride, butanamide If the suffix starts with a consonant or there are two or more of a functional group meaning di, or tri needs to be used then do not remove the the –e from the stem alkane name e.g. Propanenitrile, ethane-1,2-diol, propanedioic acid, propane-1,2,3-triol, Pentane-2,4-dione. •The functional groups take precedence over branched chains in giving the lowest number 3-methylbut-1-ene is correct and not 2-methylbut-3-ene CH2FCH2CHBrCH2CH3 3-bromo-1-fluoropentane CH2FCCl2CH2CH3 2,2-dichloro-1-fluorobutane. 2,3-dibromopentane. CH 2-bromobut-3-en-1-ol 2OHCHBrCH=CH2 C C C C C Br H H H H H Br H H H H H CHCl3 trichloromethane Where there are two or more of the same groups, di-, tri- , tetra-, penta- or hexa- are used. Note the point made above about the addition of ‘e’ to the stem H C C C C H H H H H H H O H H 4 3 2 1 code no of carbons meth 1 eth 2 prop 3 but 4 pent 5 hex 6 hept 7 oct 8 non 9 dec 10 General rules for naming carbon chains Count the longest carbon chain and name appropriately Find any branched chains and count how many carbons they contain Add the appropriate prefix for each branch chain Eg -CH3 methyl or -C2H5 ethyl –C3H7 propyl 3,5-dimethylheptane Basic rules for naming functional groups The functional group is named by a prefix or suffix. e.g. bromoethane, ethanol, propene The position of the functional group on the carbon chain is given by a number – counting from the end of the molecule that gives the functional group the lowest number. We only include numbers, however, if they are needed to avoid ambiguity. H C C C H H H H H H C H H H methylpropane Where there are two or more of the same groups, di-, tri- , tetra-, penta-, hexa- are put before the suffix/prefix. CH2FCH2CHBrCH2CH3 3-bromo-1-fluoropentane CH2FCCl2CH2CH3 2,2-dichloro-1-fluorobutane. 2,3-dibromopentane. CHCl3 trichloromethane The suffix for alkenes can go in front of other suffixes. CH 2-bromobut-3-en-1-ol 2OHCHBrCH=CH2 4 C C C C C Br H H H H H Br H H H H H Words are separated by numbers with dashes. Numbers are separated by commas. If there is more than one functional group or side chain, the groups are listed in alphabetical order (ignoring any di, tri). Halogenoalkanes Class the halogen as a substituent on the C chain and use the suffix -fluoro, -chloro, -bromo, or –iodo. (Give the position number if necessary) 2-bromobutane Alcohols These have the ending -ol and if necessary the position number for the OH group is added between the name stem and the –ol If the compound has an –OH group in addition to other functional groups that need a suffix ending then the OH can be named with the prefix hydroxy-: CH3 CH CH2 CH3 OH 1 2 3 4 Butan-2-ol H3C CH C O OH OH 2-hydroxypropanoic acid HO CH2CH2 OH H C2 HC H C2 OH OH OH If there are two or more -OH groups then di, tri are used. Add the ‘e’ on to the stem name though. Ethane-1,2-diol propane-1,2,3-triol 5 N Goalby chemrevise.org H C C C C H H H H H Br H H H C O C H H H H Aldehydes An aldehyde’s name ends in –al It always has the C=O bond on the first carbon of the chain so it does not need an extra number. It is by default number one on the chain. Ethanal C C C O H H H H H H Propanone Ketones Ketones end in -one When ketones have 5C’s or more in a chain then it needs a number to show the position of the double bond. E.g. pentan-2-one Carboxylic acids These have the ending – oic acid but no number is necessary for the acid group as it must always be at the end of the chain. The numbering always starts from the carboxylic acid end. C C O O H H C H H H H Propanoic acid If there are carboxylic acid groups on both ends of the chain then it is called a – dioic acid C C O OH O HO Ethanedioic acid Note the e in this name C C C O H H H H C H C H O H H Pentane-2,4-dione If two ketone groups then di is put before –one and an an e is added to the stem. Esters Esters have two parts to their names C C C O O C H H H H H H H H The bit ending in –yl comes from the alcohol that has formed it and is next to the single bonded oxygen. The bit ending in –anoate comes from the carboxylic acid. (This is the chain including the C=O bond) methylpropanoate O O
The characteristics of a homologous series, a series of compounds containing the same functional group.
IUPAC rules for nomenclature.
Students should be able to:
• apply IUPAC rules for nomenclature to name organic compounds limited to chains and rings with up to six carbon atoms each
• apply IUPAC rules for nomenclature to draw the structure of an organic compound from the IUPAC name limited to chains and rings with up to six carbon atoms each.
Introduction to Mechanisms The bond has broken in a process called homolytic fission. each atom gets one electron from the covalent bond When a bond breaks by homolytic fission it forms two Free Radicals. Free Radicals do not have a charge and are represented by a DEFINITION A Free Radical is a reactive species which possess an unpaired electron The Mechanism: We use curly arrows in mechanisms to show the movement of an electron pair showing either breaking or formation of a covalent bond; A curly arrow will always start from a lone pair of electrons or the centre of a bond H C C + XH H H H X HO: – δ + δ – X Y X Y Y + X: – Y: – X + 2. HETEROLYTIC FISSION: (one atom gets both electrons) OR Heterolytic fission produces IONS Cl Cl xx xx xx x x Cl Cl xx xx xx + + – two headed arrow shows movement of pair of electrons To understand how the reaction proceeds we must first understand how bonds are broken in organic mechanisms There are two ways to break a covalent bond: 1.HOMOLYTIC FISSION: Most organic reactions occur via heterolytic fission, producing ions H C C H H H H OH X Y one headed arrow shows movement of one electron. To understand a reaction fully we must look in detail at how it proceeds step by step. This is called its mechanism.The breaking of a covalent bond is shown by a curly arrow starting from the bond. The formation of a covalent bond is shown by a curly arrow that starts from a lone electron pair or from another covalent bond.
18.104.22.168 Reaction mechanisms
Reactions of organic compounds can be explained using mechanisms.
• the unpaired electron in a radical is represented by a dot
• the use of curly arrows is not required for radical mechanisms.
Students should be able to write balanced equations for the steps in a free-radical mechanism.
• the formation of a covalent bond is shown by a curly arrow that starts from a lone electron pair or from another covalent bond
• the breaking of a covalent bond is shown by a curly arrow starting from the bond.
Students should be able to outline mechanisms by drawing the structures of the species involved and curly arrows to represent the movement of electron pairs.
Structural isomers: same molecular formula different structures (or structural formulae)Functional group isomers: Compounds with the same molecular formula but with atoms arranged to give different functional groups Chain isomers: Compounds with the same molecular formula but different structures of the carbon skeleton Isomers Structural isomerism can arise from •Chain isomerism •Position isomerism •Functional group isomerism C C C H H H H H C C H H H H H H H H C C C H H H H H H C C H H H H H H C C C H H H H H C H H C H H H H pentane 2,2-dimethylpropane 2-methylbutane position isomers: Compounds with the same molecular formula but different structures due to different positions of the same functional group on the same carbon skeleton C C H H Br H C H H H H C C H H H Br C H H H H 1-bromopropane 2-bromopropane C O C H H H H H H C O H H H C H H H ethanol: an alcohol Methoxymethane: an ether C C C C C C H H H H H H H H H H H H Cyclohexane- cyclo alkane CH3CH2CH2CH2CH=CH2 hexene- alkene Note: alkene and cyclo alkanes have the same general formula. Hexene and cyclohexane have the same molecular formula but have a different functional group
Students should be able to:
• define the term structural isomer
• draw the structures of chain, position and functional group isomers
Stereoisomerism Stereoisomers have the same structural formulae but have a different spatial arrangement of atoms. Alkenes can exhibit a type of isomerism called E-Z stereoisomerism E-Z isomers exist due to restricted rotation about the C=C bond Single carbon-carbon covalent bonds can easily rotate E-Z stereoisomers arise when: (a) There is restricted rotation around the C=C double bond. (b) There are two different groups/atoms attached both ends of the double bond. C C H H C H C H H H H H C C H C H C H H H H H H But-1-ene two identical groups attached to one end of the restricted double bond – no E-Z isomers two different groups attached either end of the restricted double bond- leads to EZ isomers C H H C C H H C H H H H Z- but-2-ene E -but-2-ene These are two isomers as the lack of rotation around the double bonds means one cannot be switched to the other. Naming E-Z stereoisomers Priority Group: The atom with the bigger atomic number is classed as the priority atom Priority group side 1 Priority group side 2 If the priority atom is on the same side of the double bond it is labelled Z from the german zusammen (The Zame Zide!) If the priority atom is on the opposite side of the double bond it is labelled E from the german entgegen (The Epposite side!) Z-1,2-dichloroethene E-1,2-dichloroethene But-1-ene is a structural isomer of But-2- ene but does not show E-Z isomerism. 8 First determine the priority groups on both sides of the double bond Cl C C Cl H H Cl C C H H Cl N Goalby chemrevise.org 1. Compare the atomic number of the atoms directly attached to each side of the double bond; the group having the atom of higher atomic number receives higher priority. Cl C C Br H Cl priority priority 2. If there is a tie, consider the atoms at distance 2 from the double bond. Make a list for each group of the atoms bonded to the one directly attached to the double bond. Arranged list in order of decreasing atomic number. Compare the lists atom by atom; at the earliest difference, the group containing the atom of higher atomic number receives higher priority H C3
E–Z isomerism is a form of stereoisomerism and occurs as a result of restricted rotation about the planar carbon– carbon double bond.
Cahn–Ingold–Prelog (CIP) priority rules.
Students should be able to:
• define the term stereoisomer
• draw the structural formulas of E and Z isomers
• apply the CIP priority rules to E and Z isomers.