Alkanes and Cycloalkanes Lecture Notes PDF

Alkanes and cycloalkanes By Dr/ Ola Abdelaziz Ilentended Learninig Outcaomes (iLOs) Dr Ola Abdelaziz  Contents 1. Alkanes definition, formula and structure. 2. Physical characters of alkanes. 3. Nomenclature of alkanes. 4. Cycloalkanes properti...

Alkanes and cycloalkanes By Dr/ Ola Abdelaziz Ilentended Learninig Outcaomes (iLOs) Dr Ola Abdelaziz  Contents 1. Alkanes definition, formula and structure. 2. Physical characters of alkanes. 3. Nomenclature of alkanes. 4. Cycloalkanes properties and examples. 5. Preparation of Alkanes. 6. Reactions of Alkanes. Dr Ola Abdelaziz  Hydrocarbons Hydrocarbons are the simplest organic compounds, composed of carbon and hydrogen atoms only. There are several kinds of hydrocarbons distinguished by the types of bonding between carbons Alkanes (or saturated hydrocarbons) are hydrocarbons with only carbon-to-carbon and carbon to hydrogen single bonds Naming: alkanes naming is like counting to ten in chemistry. Methane, ethane, propane, butane is like saying 1, 2, 3, 4 in chemistry. Dr Ola Abdelaziz  Alkanes Pent- means 5. Hex- means 6. Hept- means 7. Oct- means 8. Non- means 9 (non- not nin-). Finally, dec- means 10 as in the word “decade. The sequence starts with C3H8, and a CH2 unit is added in each step moving up the series The general formula of alkanes is (CnH2n+2) where n is the number of carbon atoms in the carbon chain. Dr Ola Abdelaziz  Isomers Butan (C4H10) has 2 forms, with the same molecular formula but different arrangement in term of Isomers with the same molecular weight and different connectivity of atoms “constitutional isomers”. Isomers: compounds having the same molecular formula but different structural formulas, and therefore possessing different properties. Dr Ola Abdelaziz  Physical characters of alkanes The electronegativity difference between carbon and hydrogen (2.1 – 1.9 = 0.2) is small; therefore, the C-H bond is non-polar, meaning that the only attractions between molecules will be Van der Waals dispersion forces. Van der Waals dispersion forces will be very small for methane but will increase as the size of the molecules increase. In a larger molecule, the valence electrons are farther from the nuclei than in a smaller molecule Therefore, the melting and boiling points of the alkanes increases with the molecular size, due to the increase in intermolecular forces. Dr Ola Abdelaziz  Van der Waals Dispersion Forces ✓ It is the weakest of all intermolecular forces. ✓ It is possible for two adjacent neutral molecules to affect each other. ✓ The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom). Dr Ola Abdelaziz  Van der Waals Dispersion Forces ✓ This force is sometimes called an induced dipole-induced dipole attraction. ✓ The forces between induced dipoles are called: Dispersion forces or Van der wall interaction. Dr Ola Abdelaziz  Physical characters of alkanes ❖ Branching of an alkane chain makes the molecules more compact and brings various atoms closer which results in decreasing surface area. ✓ Straight alkanes have larger surface area which result in strong Vander Waal attraction and relatively high melting point. ✓ Branched alkanes have smaller surface area which result in minimum Vander Waal attraction. ❖ The boiling points of the alkanes decreases with branching. Dr Ola Abdelaziz  Physical characters of alkanes ✓ Cycloalkanes are similar to alkanes in their general physical properties, but they have higher boiling points and melting points, than corresponding straight chain alkanes. This is due to stronger London forces because the ring shape allows for a larger area of contact. ✓ Solubility: alkanes are non-polar molecules and therefore insoluble in water, which is polar. Alkanes are hydrophobic which dissolve in organic solvents. ✓ Densities: alkanes are less dense than water, with densities near 0.7 g/mL because they are insoluble floating on water. Dr Ola Abdelaziz  IUPA Names for alkane ✓ The International Union of Pure and Applied Chemistry (IUPAC) established a set of rules for naming alkanes. ✓ A chemical name has four parts in the IUPAC system: 1. Parent tells how many carbon atoms are in that part. 2. Prefix identify the substituent groups attached to the parent. 3. Locants give the positions of the attached substituents. 4. Suffix identifies the primary functional group attached to the parent. The suffix (–ane) to show that the compound is a saturated hydrocarbon. Dr Ola Abdelaziz  IUPA Names for alkyl groups Alkanes are quite often represented as R-H and here R stands for alkyl group. Alkyl groups are named by removing the -ane from the name of the parent alkane and adding the suffix –yl e.g., ethane gives ethyl.etc The general formula of the alkyl group is CnH2n+1. Dr Ola Abdelaziz  IUPAC Nomenclature of simple alkanes Step 1: Identify the parent hydrocarbon. (a) Find the longest continuous chain of carbon atoms in the molecule, and use the name of that chain as the parent name. The longest chain may not always be apparent from the manner of writing; you may have to “turn corners.” (b) If two different chains of equal length are present, choose the one with the larger number of branch points as the parent. Dr Ola Abdelaziz Dr Ola Abdelaziz  Step 2: Number the atoms in the longest chain (a) If the number of branch points is the same from both directions, beginning at the end nearer the first branch point, number each carbon atom in the parent chain. The first branch occurs at C3 not at C4. (b) If the first branching shows an equal distance away from both directions of the parent chain, begin numbering at the end nearer the second branch point Dr Ola Abdelaziz  Step 3: Identify and number the substituents. (a) Assign a number to each substituent to locate its point of attachment to the parent chain. (b) If there are two substituents on the same carbon, give both the same number. There must be as many numbers in the name as there are substituents Dr Ola Abdelaziz  Step 4: Write the name as a single word. ✓ Use hyphens (-) to separate the different prefixes and commas for numbers. ✓ If two or more different substituents are present, cite them in alphabetical order. ✓ If two or more identical substituents are present on the parent chain, use one of the multiplier prefixes di-, tri-, tetra-, and so forth, but don’t use these prefixes for alphabetizing. Dr Ola Abdelaziz Dr Ola Abdelaziz  IUPAC Nomenclature of branched alkanes Step 5: Name a branched substituent as though it were itself a compound ✓ In branched alkanes, the substituent on the main chain is itself branched. ✓ For example, the substituent at C6 is a branched. To name the compound fully, the branched substituent must first be named. ✓ Number the branched substituent beginning at the point of its attachment to the main chain to be 2-methylpropyl. ✓ The substituent is treated as a whole within parentheses (2-methylpropyl) including any the numerical prefix. Dr Ola Abdelaziz Dr Ola Abdelaziz  Nomenclature of alkanes 2- Common Names ✓ For alkanes beyond propane, straight-chain alkanes are sometimes indicated by the prefix n- (for normal), to distinguish them from branched-chain alkanes ✓ For example, butane is called n-butane ✓ The prefix iso is used to indicate that used when all carbons form a straight chain except one. ✓ The prefix neo refers to a substituent whose second-to-last carbon of the chain is trisubstituted (CH3)3C-. Dr Ola Abdelaziz  Common Names To assign the prefixes sec-, which stands for secondary, and tert-, for tertiary, it is important to learn how to classify carbon molecules I. If a carbon is attached to only one other carbon, it is called a primary carbon. II. If a carbon is attached to two other carbons, it is called a secondary carbon III. A tertiary carbon is attached to three other carbons IV. last, a quaternary carbon is attached to four other carbons. Dr Ola Abdelaziz  Common Names ✓ Common names for other some simpler branched-chain alkyl groups include ✓ IUPAC rules make allowance for the common names of these simple alkyl groups. Dr Ola Abdelaziz What are the two names of this compound? Dr Ola Abdelaziz  Cycloalkane ✓ When a chain contains three or more carbon atoms, the atoms can join to form ring or cyclic structures. ✓ The cycloalkane with only single bonds are named by adding the prefix cyclo- to the name of the open-chain alkane having the same number of carbons as there are in the ring. The simplest of these cyclic hydrocarbons has the formula C3H6. Each carbon has two hydrogen atoms attached and is called cyclopropane. The name for the cyclic compound C4H8 is cyclobutene Dr Ola Abdelaziz  Preparation of Alkanes 1-By the hydrogenation of alkenes or alkynes ✓ Catalytic reduction or hydrogenation achieved catalytically by shaking an alkene under hydrogen at room temperature and atmospheric pressure in presence of a transition metal catalyst as platinum or palladium ✓ The two hydrogen are added to the carbon-carbon double bond by syn-addition (addition of hydrogen from the same side). Dr Ola Abdelaziz  Perparation of Alkanes 2- Clemmensen reduction ✓ Clemmensen reduction involves the use of zinc amalgam in the presence of concentrated hydrochloric acid for the reduction of carbonyl group, of aldehydes and ketones, to methylene group 3- (Grignard 's reagents) ✓ By the hydrolysis of the alkyl magnesium halide Dr Ola Abdelaziz  4- Wurtz reaction Wurtz reaction involves heating an alkyl halide with sodium metal. The connection of two groups forms C-C σ-bond. 5- soda lime reaction Decarboxylation of carboxylic acids or their salts by heating with soda lime. Soda-lime is a mixture of Sodium hydroxide (NaOH) and Calcium oxide(CaO). Dr Ola Abdelaziz  Reactions of Alkanes 1-Oxidation (or combustion): ❖ Alkanes and cycloalkanes are quite non-reactive toward most reagents because they contain only strong sigma (σ-) bonds. The alkanes are highly combustible and can be used as fuels. Combustion is a chemical reaction when a fuel is burned to release energy Burning of alkanes and cycloalkanes in the presence of oxygen give water and carbon dioxide. Dr Ola Abdelaziz  2- Pyrolyic cracking Cracking or pyrolysis is the thermal decomposition of higher alkanes in absence of air. 3- Halogenation: ✓ The reaction is a Free radical substitution reaction where H in alkane (R-H) is substitued by Halogen X to give R-X ✓ Substitution of Y for X ✓ Free radicals are formed if a bond splits evenly, each atom getting one of the two electrons. ✓ A free radical substitution reaction involve these radicals. Dr Ola Abdelaziz The photochemical Between methane and chlorine The reaction between methane and chlorine in the presence of UV-light is a good example of a photochemical reaction brought about by light. The product is chloromethane. One of the hydrogen atoms in the methane ha been replaced by a chlorine atom, so this is a substitution reaction. All the hydrogens in the methane can in turn be replace by chlorine atoms. The mechanism involves a chain reaction. The over-all process is known as a free radical chain reaction. Dr Ola Abdelaziz  Free radical chain reaction Free radical chain reaction involves three main steps: 1- Chain initiation 2- Chain propagation 3-Chain termination 1- Chain initiation The chain is initiated (started) by UV light The role of the UV light: The ultraviolet light is simply a source of energy and is being used to break sigma bonds. In fact, the energies in UV are exactly right for breaking a chlorine molecule into chlorine free radicals. Dr Ola Abdelaziz  2- Chain propagation reactions 1. In the first propagation step, a chlorine radical abstracts hydrogen from methane. This gives hydrochloric acid and methyl radical. 2. In the second propagation step, the methyl radical reacts with more of the chlorine molecule. The methyl radical abstracts one of the chlorine atoms to form the alkyl halide product and the other chlorine atom becomes a radical Dr Ola Abdelaziz  3-Chain termination reactions ✓ These are reactions which remove free radicals from the system ✓ They are called termination because free radicals are not produced as product preventing the reaction from continuing. ✓ Radicals produced an undergo radical coupling to form a sigma bond ✓ Combining the two types of radicals produced gives three possible products Dr Ola Abdelaziz Thank you Dr Ola Abdelaziz