Lecture 3 Alkanes PDF

AN INTRODUCTION TO THE CHEMISTRY OF ALKANES Assoc.prof / Miral Ahmed Abdel Moaz 2024 Fall  Hydrocarbons ( C,H) Saturated Unsaturated i.e. contain only i.e. contain m...

AN INTRODUCTION TO THE CHEMISTRY OF ALKANES Assoc.prof / Miral Ahmed Abdel Moaz 2024 Fall  Hydrocarbons ( C,H) Saturated Unsaturated i.e. contain only i.e. contain multiple bonds single bonds (double or triple) Opened chain Cyclic Opened chain Cyclic e.g. Alkanes e.g. Cycloalkanes e.g. Alkenes and e.g. Cycloalkenes Alkynes and Aromatic compounds Representation Of Molecular Formulae Ball and stick model dash(Structural) formula CH3CH2CH2OH Condensed formula Bond line(Skeletal) formula 3 Drawing Alkanes Methane Ethane CH4 CH3CH3 Propane butane CH3CH2CH3 CH3CH2CH2CH3 n-Pentane CH3CH2CH2CH2CH3 CH3(CH2)3CH3 4  ALKANES General members of a homologous series general formula is CnH2n+2 - for non-cyclic alkanes saturated hydrocarbons - all carbon-carbon bonding is single bonds are spaced tetrahedrally about carbon atoms.  HYBRIDISATION OF ORBITALS 2p The electronic configuration of a 2 carbon atom is 1s22s22p2 2s 1 1s  HYBRIDISATION OF ORBITALS 2p The electronic configuration of a 2 carbon atom is 1s22s22p2 2s 1 1s If you provide a bit of energy you 2p 2 can promote (lift) one of the s 2s electrons into a p orbital. The configuration is now 1s22s12p3 1 1s The process is favourable because the of arrangement of electrons; four unpaired and with less repulsion is more stable  HYBRIDISATION OF ORBITALS IN ALKANES The four orbitals (an s and three p’s) combine or HYBRIDISE to give four new orbitals. All four orbitals are equivalent. Because one s and three p orbitals are used, it is called sp3 hybridisation 2s22p2 2s12p3 4 x sp3  THE STRUCTURE OF ALKANES In ALKANES, the four sp3 orbitals of carbon repel each other into a TETRAHEDRAL arrangement with bond angles of 109.5º. Each sp3 orbital in 109.5º carbon overlaps with the 1s orbital of a hydrogen atom to form a C-H bond.  Alkanes Ethane: s orbital (hydrogen) sp3 hybrids orbital (carbon) The length of the band: 1.54 A° Angle: 109.5° 10  Alkyl Groups In branched alkanes a main chain (the longest) is identified; the groups attached to the main chain are considered substitutes for the main chain and are called alkyl groups. In general terms the alkyl substitute is referred to as -R. -CH3 methyl -C2H5 ethyl -C3H7 propyl  Classification of carbon atoms Carbon atoms are classified according to the number of carbon atoms to which they are bound. Hydrogen atoms are also classified in analogy  Common Nomenclature Certain branched alkanes have common names that are still widely used today. These common names make use of prefixes, such as iso-, sec-, tert-, and neo-. The prefix iso-, which stands for isomer, is commonly given to 2-methyl alkanes. In other words, if there is methyl group located on the second carbon of a carbon chain, we can use the prefix iso-. The prefix will be placed in front of the alkane name that indicates the total number of carbons. To assign the prefixes sec-, which stands for secondary, and tert-, for tertiary, it is important that we first learn how to classify carbon molecules. If a carbon is attached to only one other carbon, it is called a primary carbon. If a carbon is attached to two other carbons, it is called a seconday carbon. A tertiary carbon is attached to three other carbons and last, a quaternary carbon is attached to four other carbons. Examples  Common: Isopentane Neopentane n- Pentane CH2Cl2 CHCl3 CCl4 Common: Methylene chloride Chloroform Carbontetrachloride IUPAC: Dichloromethane Trichloromethane Tetrachloromthane  Alkanes IUPAC Nomenclature Root prefixes # of carbon General formula: atoms CnH(2n+2) Meth- 1 Eth- 2 Prop- 3 Nomenclature But- 4 Pent- 5 root prefix based on # of Hex- 6 carbons Hept- 7 ending: -ane Oct- 8 Non- 9 Dec- 10 Example: Name this compound and write its formula Solution: Name Only single bonds → Compound is an alkane This Three carbons → Prefix prop- compound is Formula propane Count C’s and H’s → 3 C’s & 8 H’s This formula is C3H8 Root prefix # of carbon atoms You must memorize Meth- 1 the root prefixes! Eth- 2 Prop- 3 First four prefixes But- 4  Meth- monkeys Pent- 5  Eth- eat Hex- 6  Prop- peeled Hept- 7  But- bananas Oct- 8 Non- 9 Dec- 10  Straight-Chain vs. Branched Hydrocarbons Straight chain: Branched: Carbons are arranged in Some carbons branch off from a main one long chain carbon chain  Branched Hydrocarbons: Naming side chains 1. Identify the parent chain: The longest possible HC chain 2. Identify substituents (side chains or groups) Common side chains include: Alkyl groups (add –yl to prefix) Halide groups CH3- methyl Br- (bromo) CH3CH2- ethyl Cl- (chloro) CH3CH2CH2- propyl F- (fluoro) (CH3)2CH- isopropyl I- (iodo) There is a –CH3 group attached to the octane chain: It is a methyl group CH3 CH3-CH2-CH2-CH2-CH2-CH-CH2-CH3 The parent chain contains 8 carbons: It is octane If the same side chain appears more than once:  indicate how many times it appears by using prefixes:  di = 2, tri=3, tetra=4  number all carbons on the parent chain where groups are attached  separated by commas CH3 CH3-CH-CH-CH2-CH2-CH2-CH2-CH3 1 2 3 4 5 6 7 CH 8 3 This compound is: 2,3-dimethyloctane If there are different groups attached: list them alphabetically in the name ethyl groups (C3, C4) methyl group CH3 CH3 (C2) CH2 CH2 CH3 CH3 – CH2 – CH2 - CH - CH - CH - CH3 7 6 5 4 3 2 1 This compound is: 3,4-diethyl-2-methylheptane Practice: Name this molecule parent chain: nine carbons two methyl groups one ethyl group  Important Notes The common names isopropyl, isobutyl, sec-butyl, tert-butyl are approved by the IUPAC for the substituted groups. Substituent groups are cited in the name in alphabetical order, regardless of their order of occurrence in the molecule. Multiplication prefixes di, tri, ect. and structural prefixes sec., tert. written in italics and separated from the name by a hyphen) are ignored, but prefixes iso and cyclo are not! Thus “tert-butyl” precedes “ethyl”, but ethyl preceeds “isopropyl” 3-ethyl comes before 2,2-dimethyl 4-hexyl comes before 2,3-diisopropyl 3-Tert-butyl comes before 3-isopropyl 9 7 5 1 9 7 5 1 6 4 2 4 3 3 10 8 10 8 6 2 25 6-tert-Butyl-2-methyl-decane 4-Isopropyl-3-methyl-decane  Cyclic Hydrocarbons  Carbons form a ring structure  General formula (cyclic alkane): CnH2n  Prefix: cyclo- Example: cyclohexane, C6H12  Cycloalkanes are alkanes that have carbon atoms forming rings (called alicyclic compounds). Simple cycloalkanes have the formula (CH2)n, or CnH2n Nomenclature of Unsubstituted Cycloalkanes 1. Cycloalkanes with only one ring: Ring strain Bond angle 60° 90° 108° 109.5° Naming Substituted Cycloalkanes Count the number of carbon atoms in the ring and the number in the largest substituent chain. If the number of carbon atoms in the ring is equal to or greater than the number in the substituent, the compound is named as an alkyl- substituted cycloalkane i.e. use the prefix cyclo followed by the suffix indicate the number of carbon atoms. For an alkyl- or halo-substituted cycloalkane, start at a point of attachment as C1 and number the substituents on the ring so that the second substituent has as low a number as possible. Number the substituents and write the name with the substituents in alphabetical order. 28 28  Example: Cycloalkane with single substituent Br This compound is: bromocyclopentane Notice a number is not required when there is only one substituent group Example: Cycloalkane with multiple substituents Start numbering C1 with lowest alpha substituent (ethyl >> methyl) CH3 CH3 1 1 6 2 2 6 vs. 5 3 3 5 4 CH3 4 CH3 1-ethyl-3-methylcyclohexane 1-ethyl-5-methylcyclohexane Which is correct?? Example: Cycloalkane as a substituent Treat cycloalkane as a subtituent group if it is attached to a parent chain that has more carbons. This compound is: 6-methyl-3-cyclopropyldecane alpha: methyl >> propyl  Cis-Trans Isomerism In Cycloalkanes Rotation about C-C bonds in cycloalkanes is limited by the ring structure. There are two different 1,2-dimethylcyclopropane isomers, one with the two methyl groups on the same side (cis) of the ring and one with the methyls on opposite sides (trans). 32 32 Isomerism Molecules which have the same molecular formula, but differ in the arrangement of their atoms, are called isomers. Types of Isomers: 1. Constitutional (or structural) isomers differ in their structural formulas. 2. Stereoisomers differ only in the arrangement of the atoms in space. There are two types of stereoisomerism 1. Geometrical isomerism 2. Optical isomerism 33 Structural Isomers Butane and isobutane are isomers—two different compounds with the same molecular formula. Specifically, they are constitutional or structural isomers. 34 Geometrical isomers  Geometrical isomers : same molecular formula and sequence of bonded atoms, but differ in the orientation of their atoms in space  occur in organic molecules where rotation around a bond is restricted  This occurs in cycloalkanes  This occurs most often around C=C  The most common cases are around asymmetric non- cyclic alkenes 35 Geometric Isomers in cycloalkanes and alkenes A cis isomer is one in which the substituents are on the same side of the C=C or cyclic alkane A trans isomer is one in which the substituents are on the opposite sides of the C=C or cyclic alkane 36  Practice! Name these molecules A H 3C CH2 B) ) H3C CH2 CH2 CH3 CH CH3 CH CH CH2 CH3 H 2C CH2 H3C CH CH3 CH3 C) CH3 CH3 CH3 CH3 CH2 CH CH CH CH2 CH CH3 CH2 CH3  Practice: Draw the structures below A) 3-ethyl-2-methylpentane B) 3-ethyl-1,5,5-trimethylcyclohexene  PHYSICAL PROPERTIES OF ALKANES Boiling point increases as they get more carbon atoms in their formula more atoms = greater intermolecular Van der Waals’ forces greater intermolecular force = more energy to separate the molecules greater energy required = higher boiling point CH4 (-161°C) C2H6 (-88°C) C3H8 (-42°C) C4H10 (-0.5°C) difference gets less - mass increases by a smaller percentage  PHYSICAL PROPERTIES OF ALKANES Boiling point increases as they get more carbon atoms in their formula more atoms = greater intermolecular Van der Waals’ forces greater intermolecular force = more energy to separate the molecules greater energy required = higher boiling point CH4 (-161°C) C2H6 (-88°C) C3H8 (-42°C) C4H10 (-0.5°C) difference gets less - mass increases by a smaller percentage Straight chains molecules have greater interaction than branched STRUCTURAL ISOMERS OF C5H12 HIGHEST BOILING POINT LOWEST BOILING POINT “The greater the branching, the lower the boiling point”  PHYSICAL PROPERTIES OF ALKANES Melting point general increase with molecular mass the trend is not as regular as that for boiling point. Solubility alkanes are non-polar so are immiscible with water they are soluble in most organic solvents.  Preparation Of Alkanes 1- Hydrogenation of unsaturated hydrocarbon: Ni or Pd or Pt / H 2 H3C CH CH2 H3C CH2 CH3 Ni or Pd or Pt / H 2 H3C C CH H3C CH2 CH3 2- Hydrolysis of Grignard reagent Protonolysis of the alkyl Grignard reagents with water or alcohol lead to alkane 2+ Dry ether CH3CH2Br + Mg CH3CH2MgBr Grignard reagent + H3O CH3CH2MgBr 42 CH3CH3 + Mg(OH)Br 3- Reduction of alkyl halides a) By sodium metal (Coupling reaction) (Wurtz reaction) 2 H3C Br + 2 Na H3C CH3 + 2 NaBr b) By coupling of alkyl halide with lithium dialkyl cuprate (all kinds of alkanes) (CH3CH2)2CuLi + CH3Br CH3CH2CH3 43  Reactions Of Alkanes Introduction - fairly unreactive; (old family name, paraffin, meant little reactivity) - have relatively strong, almost NON-POLAR, SINGLE covalent bonds - they have no real sites that will encourage substances to attack them 1. Combustion - react with oxygen in an exothermic reaction complete CH4(g) + 2O2(g) ——> CO2(g) + 2H2O(l) combustion incomplete CH4(g) + 1½O2(g) ——> CO(g) + 2H2O(l) combustion the greater the number of carbon atoms, the more energy produced BUT the greater the amount of oxygen needed for complete combustion. Handy tip When balancing equations involving complete combustion, remember... every carbon in the original hydrocarbon gives one carbon dioxide and every two hydrogen atoms gives a water molecule.  BREAKING COVALENT BONDS There are 3 ways to split the shared electron pair in an unsymmetrical covalent bond. UNEQUAL SPLITTING produces IONS known as HETEROLYSIS or HETEROLYTIC FISSION EQUAL SPLITTING produces RADICALS known as HOMOLYSIS or HOMOLYTIC FISSION If several bonds are present the weakest bond is usually broken first Energy to break bonds can come from a variety of energy sources - heat / light In the reaction between methane and chlorine either can be used, however... In the laboratory a source of UV light (or sunlight) is favoured.  Reactions Of Alkanes Chemically alkanes are very unreactive and stable at room temperature towards acids , bases and most reactive metals. Despite their relative inertness ( thus they known as paraffines i.e lacking affinity) , alkanes undergo several important reactions that are discussed in the following section. 2- Halogenation: Halogenation is the replacement of one or more hydrogen atoms in an organic compound by a halogen (fluorine, chlorine, bromine or iodine). The halogenation of an alkane appears to be a simple free radical substitution reaction in which a C-H bond is broken and a new C-X bond is formed; the reaction takes place in presence of heat or UV light ( no reaction in the dark) Heat RH + X2 RX + HX X = Cl or Br or UV light Alkyl halide 46 Reactivity: X2=Cl2>Br2  CHLORINATION OF METHANE OVERVIEW Initiation Cl2 ——> 2Cl radicals created Propagation Cl + CH4 ——> CH3 + HCl radicals used and Cl2 + CH3 ——> CH3Cl + Cl then re-generated Termination Cl + Cl ——> Cl2 radicals removed Cl + CH3 ——> CH3Cl CH3 + CH3 ——> C2H6 Summary Due to lack of reactivity, alkanes need a very reactive species to persuade them to react Free radicals need to be formed by homolytic fission of covalent bonds This is done by shining UV light on the mixture (heat could be used) Chlorine radicals are produced because the Cl-Cl bond is the weakest You only need one chlorine radical to start things off With excess chlorine you get further substitution and a mixture of chlorinated products  CHLORINATION OF METHANE Initiation Cl2 ——> 2Cl RADICALS CREATED The single dots represent UNPAIRED ELECTRONS During initiation, the WEAKEST BOND IS BROKEN as it requires less energy. There are three possible bonds in a mixture of alkanes and chlorine. 412 348 242 Average bond enthalpy kJ mol-1 The Cl-Cl bond is broken in preference to the others as it is the weakest and requires requires less energy to separate the atoms.  CHLORINATION OF METHANE Propagation Cl + CH4 ——> CH3 + HCl RADICALS USED and Cl2 + CH3 ——> CH3Cl + Cl then RE-GENERATED Free radicals are very reactive because they want to pair up their single electron. They do this by abstracting a hydrogen atom from methane; a methyl radical is formed The methyl radical is also very reactive and attacks a chlorine molecule A chlorine radical is produced and the whole process can start over again  CHLORINATION OF METHANE Termination Cl + Cl ——> Cl2 RADICALS REMOVED Cl + CH3 ——> CH3Cl CH3 + CH3 ——> C2H6 Removing the reactive free radicals brings an end to the reaction. This is not very likely at the start of the reaction because of their low concentration.  CHLORINATION OF METHANE RADICALS Initiation PRODUCED Propagation RADICALS USED AND REGENERATED Termination RADICALS REMOVED If there is one type of the carbon atoms in the molecule (e.g. methane and ethane) H Cl2 UV light H C H + CH3Cl + CH2Cl2 + CHCl3 + CCl4 + 4HCl excess or Heat H Cl2 Cl2 Cl2 Cl2 CH4 CH3Cl CH2Cl2 CHCl3 CCl4 UV UV UV UV If there are different types of carbon atoms in the molecule (Selectivity issue) When alkanes larger than ethane are halogenated, isomeric products are formed. The preferred order for the hydrogens to be substituted is 3° then 2° then 1°. Thus chlorination of propane gives both 1-Bromopropane a s minor product and 2- Bromopropane as major mono-chlorinated product. 2° ° Br 1° 1 UV light + H3C CH3 + Br2 or Heat Br H3C CH3 H3C CH2 52 Propane Major Minor  Reactions Of Cycloalkanes Less stable rings H2/ Ni Br HBr Br2 Br Br H2O/ conc. H2SO4 HO More stable 5 and 6 rings CH3 CH3 Br2/UV or Heat Br Cl Cl 2/heat or UV 53