CHEMISTRY OF HYDROCARBONS (1).docx

**CHEMISTRY OF HYDROCARBONS** **A. ALKANES** - General formula - Subdivision -- Linear - Cycloalkanes \- Branched - Why are they called paraffins? - What are the chemical properties of alkanes - What are the physical properties of alkanes - What is the laboratory preparation...

**CHEMISTRY OF HYDROCARBONS** **A. ALKANES** - General formula - Subdivision -- Linear - Cycloalkanes \- Branched - Why are they called paraffins? - What are the chemical properties of alkanes - What are the physical properties of alkanes - What is the laboratory preparation of an alkane (eg. Methane) - Their importance. - Difference from other hydrocarbons **1. General Formula** The general formula of alkane is C~n~H~2n~ +2 The first 6 alkanes are; Methane CH~4~ Ethane C~2~H~6~ Propane C~3~H~8~ Butane C~4~H~10~ Pentane C~5~H~12~ Hexane C~6~H~14~ **2. What are the Subdivision** **a. Linear Alkanes** These are the straight chain alkanes e.g Methane CH~4~, Ethane C~2~H~6~ **b.** The Branched Alkanes A branched chain alkane is alkane which has alkyl group bonded to one or more carbon atoms in the chain e.g CH~3~ ׀ CH~3~ ̶ CH ̶ CH~2~ ̶ CH~3~ 2 methy butane **c. They Cycloalkanes** In organic chemistry, the cycloalkanes are the monocyclic saturated hydrocarbons. In otherwords, a cycloalkane consists only of hydrogen and carbon atoms arranged in a structure containing a single ring and all the c-c bonds are single. e.g. Alkanes are the simplest and the least reactive hydrocarbon species containing only carbon and hydrogens. **3. Why are alkanes called Paraffins** Alkanes are otherwise called Paraffins because they have little affinity for other compounds. They are saturated hydrocarbons and lack any other functional groups. **4. What are the Chemical Properties of Alkanes** Alkanes are not very reactive family of compounds. They do not react with the common laboratory reagents such as acids, alkalis, common oxidizing or reducing reagents. Hence the word paraffin (which means little affinity). Alkanes generally exhibit similar reactions when they react; i. Methane burns in plentiful supply of air to form carbondioxide and water CH~4~ + 2O~2~ → CO~2~ + H~2~O ∆H = -890kjmol^-1^ ii. When methane is mixed with air and ignited, it explodes violently. This accounts for most explosions in coal mines. Combustion is one of the very few chemical reactions of alkanes. Combustion is an exothermic reaction. Alkene are therefore useful sources of heat energy. Thus, hydrocarbons generally constitute the most important energy source for today's modern technology. iii. Chemical test -- Unlike alkane and alkyne, alkanes neither decolorize bromine water nor potassium teraoxomangnate (VII) solution. iv. Alkanes react with chlorine or bromine in the presence of sunlight, the reactions is known as pholocatalysis e.g. This is a substitution reaction whereby one hydrogen atom in methane is substituted by a chlorine atom. This substitution can go further to substitute all the hydrogen atoms in methane by chlorine atoms. CH~4~ + CL~2~ → CH~3~CL + HCL (Chloromethane) CH~3~CL +CL~2~ → CH~2~CL~2~ + HCL (Dichloromethane) CH~2~CL~2~ +CL~2~ → CHCL~3~ + HCL (Trichloromethane) CHCL~3~ +CL~2~ → CCL~4~ + HCL (Tetra-chloromethane) Halogen substituted alkanes are very useful products. For example, trichloromethane CHCL~3~ (chloroform) was about the first known anaesthetic. Triodomethane CHI~3~ (iodoform) is a germicide which kills harmful skin bacteria. Tetrachloromethane, CCL~4~ (Carbon tetrachloride) is a very useful organic solvent. Both iodoform and carbon tetrachloride are poisonous on exposure to light and air especially in confined places. Alkane have been made to undergo a number of useful reactions by the use of special conditions such as high temperatures and catalysts. **Physical, Properties of Alkanes** - Solubility properties - Boiling point properties - Melting point properties **a. Solubility Properties** i. Due to the small difference in electro-negativity between carbon and hydrogen as well as the covalent property of the C-C bond or the C-H bond, alkanes are commonly non-polar kind of molecules. ii. As we observe normally, polar molecules seem to be soluble in polar solvents. Alkanes are hydrophobic in nature hence insoluble in water. iii. Alkanes are soluble in organic solvents because the energy needed to overcome the established vanderwaal forces and to create new vanderwaal forces is quite comparable. **b. Boiling Point Properties** i. As the molecular weight of the alkanes increases, the boiling point also increases. ii. Structural isomers have a lesser boiling point as compared to their equivalent straight chain alkanes. **c. Melting Point Properties** i. Melting point increases as the molecular weight is increased. ii. Due to the fact that the higher alkanes are in the solid state and hence more difficult to overcome the intermolecular force of attraction, the melting point becomes higher with higher alkanes. **Laboratory preparation of an Alkane (e.g. methane)** **Importance (Uses) of Alkanes** i. They are commercially very important, being the principal constituents of gasoline and lubricating oils and are extensively employed in organic chemistry though the role of pure alkanes (such as hexane) is delegated mostly to solvents (it can extract oils from seeds leaving the cake e.g castor oil and recovered by evaporation) ii. As fuel, methane is the main component of natural gas while butane is the main component of camping gas and lighter fuel. Octane is an important component of petrol. iii. Through the cracking process, alkanes are used to produce very useful unsaturated hydrocarbons such as ethane, C~2~H~4~. **Difference of Alkane from other hydrocarbons (such as Alkenes and Alkynes)** **The difference lies in its lacks of unsaturated**: Alkanes are saturated hydrocarbons as they contain no double or triple bonds which are highly reactive in organic chemistry. Their lack of reactivity under most laboratory condition makes them relatively uninteresting, even though they are very important component of organic chemistry. **Order of Strength of Hydrocarbons** Alkynes \> alkenes \> alkanes Alkynes are the strongest because of triple bonds. so the bond enthalpy (energy required to break the bond) is maximum. **Order of Reactivity of the Hydrocarbons** Alkanes \< alkanes \< alkynes Alkynes are most reactive due to larger electron density between C -- C triple bond in electrophilic reactions. The π electron density easily attracts electrophilies making alkynes most reactive. **Order of stability (Thermodynamically)** Alkanes \> alkanes \> alkynes Reasons: Alkenes and alkynes have more electron densities due to π bond and they are susceptible to attack by electrophiles **B. ALKENES** - What are Alkenes - Why Alkenes are called Olefins - Points to note about Alkenes - Laboratory Preparation of an alkene (e.g ethane) - Chemical tests for Alkenes - Addition reactions of Alkenes - Uses of Alkenes. **1. What are Alkenes** Alkenes form a homologous series with the general formula CnH~2n~. They posses a double bond between two of their carbon atoms. The simplest member of the alkenes is ethane. The C = C double bond makes alkenes far more reactive than the alkanes. Unlike the alkanes, the alkenes are unsaturated hydrocarbons. The member with n = 1 does not normally exist but occurs as a short-lived force radical in some reactions. The free radical is CH~2~. **Why alkenes are called Olefins** They are called Olefins because the lower members of alkenes form oily products when they are treated with chlorine or bromine. The name was derived from the original French "gas Olefiant" meaning oil forming gas. **Points to note about Alkenes** - All alkenes have names which end in -- ene - All alkenes contain C = C bond called ethylenic bond - Butene C~4~H~8~ possesses three Isomers. Pentene posses four isomers. - Isomerism in alkenes is brought about by shifting the positions of double bond along the carbon chain and also by the branching of the carbon chain. - Isomers of butene~(g)~ and Pentene~(L)~ have numbers in their names. The numbers show the location of the double bond. Thus, the first isomer of butene C~4~H~8~ is called but-1-ene. CH~2~ = CH -- CH~2~ -- CH~3~ But-1-ene. The second isomer of butene is but-2-ene CH~3~ -- CH = CH -- CH~3~ But-2-ene. In the first case, the double bond is between carbon atoms 1 and 2 while in the second case, the double bond is between carbon atoms 2 and 3. - The same rule is observed in naming other isomers of alkenes. - Geometrical Isomerism: In addition to structural isomerism already discussed, alkenes also exhibit geometrical isomerism since free rotation cannot take place about the double bond, two additional isomers are possible in the butene molecule C~4~H~8~. The methyl groups of butene can be situated either on the same side of the double bond or on opposite sides. The two geometrical isomers of butene are known as the cis-form and trans-form. - The cis-form has the structure - The trans-form isomer of butane has the following structure. So, altogether butane has four Isomers. - The lower alkenes are colourless flammable gases. For example ethene C~2~H~4~ has a boiling point of -- 104^0^C at atmospheric pressure while propene C~3~H~6~ has a boiling point of -- 48^0^C at atmospheric pressure. - Boiling points rise with increase in molar mass. For example Pentene and hexene are liquids at ordinary temperatures. - Gaseous alkenes dissolve only slightly in water but are moderately soluble in ethanol and ether. - Alkenes do not occur naturally because of their high reactivity, lower members of alkenes are manufactured by cracking petroleum fractions. **Laboratory preparation of an Alkens (e.g Ethene)** Alkenes are generally prepared by dehydration of corresponding alcohols. If an alcohol is heated with a suitable catalyst, one molecule of water is eliminated from each molecule of the alcohol. (ethanol) (ethane) **Chemical tests for Alkenes** - Ethene burns readily in air forming carbondioxide and water. C~2~H~4~ + 3O~2~ → 2CO~2~ + 2H~2~O The reaction is exothermic - Add a few drops of bromine water in a gas jar of ethane, the brown colour of bromine is decolorized. This is usual test for an unsaturated compound. - Add a few drops of acidified KMNO~4~ to the gas jar of ethane, the purple colour of acidified potassium tetraoxomagnates (VII) turns colourless. This is another useful test for an unsaturated compound. - The carbon-carbon double bond in alkenes consists of a strong and a weak bond. The weak bond accounts for the high reactivity of alkenes. **Addition Reactions of Alkenes** Alkenes normally undergo addition reactions on which their molecules add on complete molecules of other substances. The weaker of the carbon-carbon double bond in alkene is broken and the different parts of the added molecule become linked to the two carbon atoms as shown. - Due to the tendency to undergo addition reaction, alkenes are known as unsaturated hydrocarbons. An unsaturated compound is a compound which combines directly with other substances forming in each case, a single new compound. In contrast, alkanes are saturated hydrocarbons and their characteristic mode of reaction is by substitution. a. Addition of hydrogen (hydrogenation) (ethane) (ethane) The above reaction is an important stage in the conversion of vegetable oils into margarine. b. Addition of halogens (Halogenation) The order of reactivity of halogens with alkenes is FL \> CL \> Br \> I The reaction with fluorine is rather explosive with chlorine and Bromine, the reaction is very rapid at room temperature to give 1, 2 dichloroethane and 1,2 dibromoethane respectively. The halogen is added across the double bond. (1, 2 dichloroethane) (Brown) Colourless (1,2 dibromoethane) c\) Addition of hydrogen halides (hydrohalogenation) The reactivity order of hydrogen halides with alkenes is HI \> HBr \> HCL \> HF Ethene combines with hydrogen iodide and hydrogen bromide at room temperature with anhydrous aluminium III chloride acting as a catalyst. (Chloroethane) Addition reaction with hydrogen fluoride (HF) can only take place under pressure. d\) Addition of water (Hydration) When water is chemically added to a compound, the reaction is known as hydration. Hydration of alkenes is indirectly achieved with tetraoxosulphate (VI) acid and the result is the formation of alcohols. (ethylhydrogen tetraoxosulphate (vi) e\) Addition of Oxygen (Oxidation) Baeyer's reagent is a cold KMnO~4~ alkaline solution. This reagent oxidizes ethane to ethane-1,2 diol (ethylene glycol) (Note: Acidified KMnO~4~ is a very strong oxidizing agent, capable of oxidizing many compounds completely to carbon dioxide and water. (ethane 1, 2, diol) **\ ** **Uses of Alkenes** - Alkenes are starting materials for very large numbers of organic compounds including solvents, plastics, detergents etc. - Ethene in particular is used in hastening fruit ripening. **Industrial manufacture of Ethene** Ethene is produced commercially b cracking gas oil fraction of petroleum at about 12 atmospheric pressure and 500^0^C. A mixture of lighter molecules of alkanes, ethane and other alkenes are produced. C~17~H~36~ → C~8~H~18~ + 2C~2~H~4~ + C~3~H~6~ (decaheptane) (Octane) (ethane) (Propene) **\ ** **C. ALKYNES** - What are Alkynes - Laboratory preparation of an alkyne (e.g ethyne) - Properties of Ethyne - Chemical test for Alkynes (e.g ethyne) - Addition reaction of alkynes (eg Ethyne) - Uses of Ethyne. **What are Alkynes** The alkynes form the second series of unsaturated hydrocarbons. The functional group of alkynes is ̶ C ≡ C ̶ in otherwords, an alkyne possesses a triple bond between two of the carbon atoms. The simplest member of alkynes is ethyne C~2~H~2~ otherwise called **acetylene**. - General formula: Alkynes form a homologous series with the general formula CnH~2~n-2 The following are the first four members of the series. 1. Ethyne (acetylene) C~2~H~2~ 2. Propyne C~3~H~4~ 3. Butyne C~4~H~6~ 4. Pentyne C~5~H~8~ **Laboratory Preparation of an alkyne (e.g ethyne)** i. CaC~2~ + 2H~2~O → C~2~H~2~ + Ca(OH)~2~ (ethyne) ii. CH~2~BrCH~2~Br + 2KOH → CH ≡ CH + 2KBr + H~2~O 1,2 dibromoethane. **Properties of Ethyne** - Ethyne decolorizes the brown colour of bromine water (a test for unsaturation) - Ethyne decolorizes the purple colour of acidified KMnO~4~ solution (another test for unsaturation). - Ethyne burns in air with a luminous smoky flame forming carbon dioxide and water. The smoky flame is due to incomplete combustion because of the high carbon content of the gas which is about 92.3% - Ethyne burns in oxygen with a smokeless flame, the well known oxyacetylene flame which has a very high temperature. Combustion of ethyne is said to be complete. 2C~2~H~2~ + 3O~2~ → 4CO~2~ + 2H~2~O - Ethyne produces a white precipitate of silver dicarbide with silver trioxonitrate (V) solution. 2AgNO~3(ag)~ + C~2~H~2~ → Ag~2~C~2(s)~ + 2HNO~3(ag)~ (white precipitate of silver dicarbide) - The carbon -- carbon triple bond in alkynes consists of one strong bond and two weaker bonds. The two weaker bonds account for the higher reactivity of alkynes. - Alkynes like alkenes are unsaturated and both characteristically undergo addition reaction with electrophilic agents. As a result of the triple bond in ethyne, it is more unsaturated than ethane and forms addition products with two or four univalent atoms or groups. The addition reactions usually take place in two stages; i. The bond between the carbon atom change first into a double bond and ii. Finally into a single bond. **\ ** **Chemical test for Alkynes** - Test the gas with bromine water and acidified KMnO~4~ like ethane, ethyne decolorizes both bromine water and acidified KMnO~4~. **Addition Reactions of Ethyne** - Hydrogenation When a mixture of ethyne and hydrogen is passed over finely divided Ni catalyst at 140^0^C, ethene and ethane are produced. - Halogenation Reactive order of halogens with alkynes F~2~ \> CL~2~ \> Br~2~ \> I~2~ Fluorine and chlorine react explosively with alkyne at room temperature. H ̶ C≡ C ̶ H + CL~2~ → - Hydrohalogenation (Addition of hydrogen halides) The reactivity order of hydrogen halides with alkynes is HI \> HBr \> HCL with hydrogen iodide and hydrogen bromide, addition reactions occur at room temperature and also in two stages; H ̶ C ≡ C ̶ H + HBr → **Uses of Ethyne** 1\. Ethyne is employed in oxyacetylene flame used in welding and in cutting steal. 2\. Many non flammable industrial solvents are prepared from ethyne e.g trichloroethene (CHCL = CCL~2~) and tetrachloroethene (CCL~2~ = CCL~2~). Trichloroethene is used for extracting oil from seeds while tetrachloroethene is used in the dry cleaning industry. 3\. Ethyne is a good starting material for the synthesis of other important organic compound e.g ethanoic acid CH~3~COOH. **FUNCTIONAL GROUPS** ABBREV 1. R-OH Alcohol 2. R -- NH~2~ Amine 3. R -- SH Thiol 4. R -- CL Chloride 5. RCHO Aldehyde 6. RCOR Ketone 7. RBr Bromide RI Iodide 8. Amide 9. RNH~2~ Amine 10. RCOCL Acid Chloride 11. RCOOH Carboxylic acid 12. RCOOCOR Anhydride 13. RCOOR Ester 14. RCHNR Imine 15. RNOO Nitrogroup 16. ROP Ether e.g CH~3~OCH~3~ ~Methoymethane\ (dimethylether)~ **\ ** 17\. Urea H~2~NCO(NH~2~) or CO (NH~2~)~2~ (Carbamide) 18\. RH Alkyl halides 19\. R ̶ C ≡ N Nitriles **\ ** **D. ALDEHYDES AND KETONES** General formula Aldehydes and Ketones are two related categories of organic compounds in that both contain the carbonyl group. The difference between aldehydes and Ketones is the placement of the carbonyl group within the molecule. **Definition** An Aldehyde is an organic compound in which the carbonyl group is attached to a carbon atom at the end of a carbon chain. A Ketone is an organic compound in which the carbonyl group is attached to a carbon atom within the carbon chain. Examples: For aldehydes, the R group may be a hydrogen atom or any length carbon chain. Aldehydes are named by finding the longest continuous chain that contains the carbonyl group. Change the name of the --e at the end of the chain to --al. For Ketones, R and R^1^ must be carbon chains of either the same or different lengths. The step for naming ketones followed by two examples are shown below; 1. Name the parent compound by finding the longest continuous chain that contains the carbonyl group. Change the --e at the end of name of the alkane to --one. 2. Number the carbon atoms in the chain in a way so that the carbonyl group has the lowest possible number. 3. Add the numerical prefix into the name before the name of the ketone. 4. Use a hyphen between the number and the name of the ketone. Both Aldehydes and ketones have lower boiling points than alcohols. **FIVE GENERAL REACTIONS OF ALDEHYDES AND KETONES** - Nucleophilic addition - Reduction reaction - Oxidation reaction - Halogenations - Reaction with alkalis **EXAMPLES** **ALDOL CONDENSATION** Aldol condensation can be defined as an organic reaction in which enolate ion reacts with a carbonyl compound to form β hydroxyketone or β hydroxyl aldehyde, followed by dehydration to give a conjugated enone. Aldol condensation plays a vital role in organic synthesis creating a path to form carbon-carbon bonds. **General Aldol Condensation** **\ ** **E. NITRILES (R ̶ C≡ N)** The prefix cano -- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds including methylcyanoacrylate used in superglue and nitrile rubber, a nitrile containing polymer used in latex-free laboratory and medical gloves. Nitrile rubber is also widely used as automotive and other seals since it is resistant to fuels and oils. Organic compounds containing multiple nitrile groups are known as cyanocarbons. The functional group is ̶ C ≡ N Inorganic compounds containing the ̶ C ≡ N group are not called nitriles, but cyanides instead. **\ ** **F. ALCOHOLS** **General formula ROH** Where R is an alkyl group Examples are; CH~3~OH Methanol C~2~H~5~OH Ethanol C~3~H~7~OH Propanol **Production of Alcohol** Alcohol is commonly produced by fermentation. Fermentation is the decompositions of complex organic compounds into simpler components through the agency of yeast. The starting material for the production of alcohol is starch. Guinea corn is one of the main sources in West Africa. Ethanol can be obtained from other starch-containing substances such as cassava, maize, potatoes etc. **Industrial Production of Alcohol** - The starch-containing substances are crushed and treated with steam to extract the starch from them. - Malt made from partially -- germinated barley (often called malted Barley is added and then kept warm for about one hour. - The enzyme diastase present in the malt, catalyses the conversion of starch into maltose. Maltose - Yeast is then added. The yeast contains two enzymes namely maltase and zymase. - The maltase catalyses the conversion of maltose to glucose while zymase catalyses the conversion of glucose to alcohol (ethanol). - The chemistry of the reaction is as follows; glucose ethanol The ethanol is separated from the main solution by fractional distillation. **There are three types of Alcohol** a. Primary alcohol (1^0^ alcohol). They contain one alkyl group and denoted by the formula RCH~2~OH ethanol. b. Secondary alcohol (2^0^ alcohol). They contain two alkyl groups and denoted by the formula R~2~CHOH. e.g CH~3 ~ ̶  CH  ̶ CH~3~ 2 propanol c. Tertiary alcohol (3^0^ alcohol). They contain three alkyl groups and denoted by the formula R~3~COH. E.g 2 methyl-propan-2-ol. 2 methyl propan-2-ol. **LUCAS TEST** To distinguish 1^0^, 2^0^ and 3^0^ Alcohols Lucas Reagent -- Solution of anhydrous ZnCL~2~ in Conc HCL reaction here is Nucleophilic substitution. In the reaction, the chloride is ZnCl~2~ bond is replaced by OH group from alcohol. **REACTIONS** 1^0^ alcohol do not react readily at room temperature with the added Lucas reagent whereas 3^0^ alcohols react immediately. 1^0^, 2^0^ and 3^0^ alcohols react with the Lucas reagent to form the chloroalkane at different rates. **LUCAS TEST** 1^0^ Alcohols - The solution remains colourless unless it is subjected to heat. The solution forms an oily layer when heated e.g 1-Pentanol. 2^0^ Alcohol - The solution turns turbid and forms an oily layer in 3 to 5 minutes e.g 2 Pentanol 3^0^ Alcohol - The solution turns turbid and forms an oily layer immediately e.g 2 methyl, 2 butanol 2 methyl, 2 butanol.