Alkanes PDF - Aliphatic Hydrocarbons

10/13/2024 Aliphatic hydrocarbons (alkanes) Marwa Ehab Abdelaziz, PhD Lecturer of Pharmaceutical Chem...

10/13/2024 Aliphatic hydrocarbons (alkanes) Marwa Ehab Abdelaziz, PhD Lecturer of Pharmaceutical Chemistry Faculty of Pharmacy Alexandria University 1 Physical Chemical Nomenclature Preparation properties reactions 2 1  10/13/2024 Alkanes Alkanes are saturated hydrocarbons that consist of single-bonded carbon and hydrogen atoms only. Alkanes have the general molecular formula CnH2n+2. Alkanes are commonly referred to as paraffins and are a major component of natural gas and petroleum. Alkanes can be subdivided into the following three groups: the linear straight-chain alkanes, branched alkanes, and cycloalkanes ▪ Tetrahedral orientation of groups is the rule for the carbon atoms of all alkanes and cycloalkanes (sp3 hybridization). 3 Nomenclature Alkanes have a specific prefix and end with suffix ane An alkyl group is formed by removing one hydrogen from the alkane chain. The removal of this hydrogen results in a stem change from -ane to -yl. 4 2  10/13/2024 Nomenclature IUPAC Nomenclature 1- Select the longest connected chain of hydrocarbons with the greatest number of substituents (most branched). 2- Number the longest chain from the end nearest to the branch so that substituent would take lowest possible no. 2-Ethylhexane X 5-Methylheptane X 3-Methylheptane 5 3- When two or more substituents are identical, use the prefixes di-, tri-, tetra- etc. 4- After applying the previous rules, make sure that substitutes are written in alphabetical order. 2,5,6-Trimethylheptane X 2,6-Dimethyl-4-ethylheptane X 2,3,6-Trimethylheptane 3- Ethyl-2,6-dimethylheptane 6 3  10/13/2024 Examples for nomenclature 4-Ethyl-3-methyloctane 4-Ethyl-3,5-dimethyloctane 5-Ethyl-2,4,6-timethyloctane 7 Nomenclature of cycloalkanes Monocyclic compounds Cycloalkanes with only one ring 8 4  10/13/2024 Classification of alkyl groups 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 secondary carbon. A tertiary carbon is attached to three other carbons and last, a quaternary carbon is attached to four other carbons. 9 The prefixes sec-, which stands for secondary, and tert-, for tertiary. The prefix iso-, which stands for isomer, is commonly given to 2-methyl alkanes. The prefix neo- refers to a substituent whose second-to-last carbon of the chain is trisubstituted (has three methyl groups attached to it). A neo-pentyl has five carbons total. H3C CH3 CH X H3C C CH2 H3C CH3 X isopropyl neopentyl 10 5  10/13/2024 Physical properties The first four alkanes (C1-C4) are gases at room temperature, (C5-C7) are liquids and solids do not begin to appear until about C17H36. Branched isomers have lower boiling points than straight chain isomers. (Branching in branched isomers decreases the surfaces area, weakens the IMFs, and lowers the boiling point) Alkanes and cycloalkanes are almost totally insoluble in water because of their very low polarity and their inability to form hydrogen bonds. 11 Preparation 1) hydrogenation of alkenes and alkynes Catalytic hydrogenation Example: 12 6  10/13/2024 Preparation reactions 2) Reduction of alkyl halides Example: 13 Chemical reactions Halogenation of an alkane 14 7  10/13/2024 Halogenation of an alkane Halogenation of an alkane is a substitution reaction. Multiple substitutions almost always occur in the halogenation of alkanes. 15 Halogenation of an alkane Mechanism of action (free radical mechanism) 1) Initiation 2) Propagation 16 8  10/13/2024 3) Termination CH3Cl CH3CH3 Cl2 17 Factors affecting rate of halogenation Many different factors affect both the initiation and propagation of free radicals, which has a significant impact on free radical reactions. 1) Nature of the Substrate 2) Type of Halogen (in Halogenation Reactions) 3) Presence of Light, Heat, or Radical Initiators 18 9  10/13/2024 Factors affecting rate of halogenation 1) Nature of the Substrate Stability of the Hydrogen Atom ▪ 3° hydrogen atoms of an alkane are most reactive, 2° hydrogen atoms are next most reactive, and 1° hydrogen atoms are the least reactive. ▪ Alkanes with more branching tend to favor radical formation at the most stable carbon atom. Tertiary radicals are more stable due to hyperconjugation from neighboring alkyl groups. 19 Example H3C H2 H2 C Cl2 C CH Cl H3C CH2Cl + H3C CH3 h H3C n-propane n-propyl chloride isopropyl chloride 45% 55% 20 10  10/13/2024 Factors affecting rate of halogenation 2) Type of Halogen In free radical halogenation reactions, different halogens react with varying reactivity: Fluorine (F₂): Extremely reactive, often explosive, and less selective. Iodine (I₂): Usually does not participate in free radical halogenation due to its low reactivity. Bromine (Br₂): Less reactive but much more selective, favoring more stable free radicals (e.g., tertiary radicals over secondary or primary). Chlorine (Cl₂): Moderately reactive and moderately selective. It reacts more quickly but with less selectivity. 21 Factors affecting rate of halogenation 3) Presence of Light, Heat, or Radical Initiators Light (hv): Ultraviolet (UV) light or visible light can supply energy to break bonds and initiate radical formation. Heat: Increases the kinetic energy of molecules, allowing for the cleavage of bonds and the initiation of free radical reactions. Conditions for chlorination: uv light (hʋ) or heat 250-400 22 11  10/13/2024 Questions 1) Give the IUPAC nomenclature for the following compound 2)......... is the most reactive toward halogenation (substitution) 23 References Organic Chemistry, fourth edition, Paula Yurkanis Bruice. Fundamentals of Organic Chemistry, fifth edition, T.W. Graham Solomons. Organic Chemistry, 6th edition, R.T. Morrison and R.N. Boyd. 24 12  10/13/2024 25 13