Hydrocarbons

Organic Chemistry Organic chemistry is the study of carbon-containing compounds – carbon chemistry Until early 19 th century, it was thought that all organic compounds were formed from living organisms This changed when Friedrich Wöhler, “father of organic chemistry”, synthesized urea in the laboratory in 1828 (Urea is a waste product of protein breakdown in the body)

The Chemistry of Carbon Number of carbon-containing compounds are very high – Why? 1) Carbon can form (up to 4) stable covalent bonds with other carbon atoms 2) Carbon atoms can form stable bonds with other elements (oxygen, nitrogen, sulfur, halogens) 3) Carbon can form double and triple bonds with other carbon atoms to form various organic compounds with different properties

Groups of Organic Compounds

Hydrocarbons (only C and H) Aliphatic (open chain and ring without conjugation) Alkanes Cycloalkanes Alkenes Alkynes Aromatic (ring with conjugation)

Alkanes Alkanes are saturated hydrocarbons – they contain only carbon-carbon and carbon- hydrogen single bond C n H 2n+2 is the general molecular formula for alkanes All hydrocarbons are nonpolar molecules – not soluble in water They are soluble in nonpolar organic solvents They have relatively low melting and boiling points and lower density then water As the hydrocarbon chain gets longer, melting and boiling point increases

Alkyl groups Alkyl groups are alkanes with one fewer hydrogen atom Methane, methyl propane, propyl Used to indicate hydrocarbons as substituents

Table for Common Alkane Parent Chains

Naming Alkanes Many common organic compounds are named historically by the name of the person who discovered it or by a name that reflects the source of the compound Penicillin (Penicillium notatum), aspirin As the number of organic compounds increased, the need for a systematic nomenclature was realized IUPAC (The International Union of Pure and Applied Chemistry) – the organization responsible for establishing and maintaining a universal nomenclature system (IUPAC Nomenclature System) Penicillin core aspirin

IUPAC Rules for Naming Alkanes 1) Determine the name of the parent compound – the longest continuous carbon chain in the compound (use the table) pent – ane (related to number + ane) 2) Number the carbons in the parent compound from one end such that the first substituent encountered takes the lowest possible number 3) Name and number each of the substituents on the parent compound (alkyl or halogen substiuents) 4) If the same substituent occurs more then once, a separate position number is given for each (di-, tri-, tetra-, etc.) 5) Place the name of the substituents in alphabetical order before the name of the parent compound – numbers are separated by commas and numbers are separated from names by hyphen (-) 2,3-Dimethylhexane 3-Fluoro-2,4-dimethylhexane (halogens are placed before alkyl substituents)

Constitutional Isomers Isomers that have the same molecular formula, but different connectivity of its atoms They have different physical and chemical properties

Cycloalkanes Saturated hydrocarbons that have a ring structure are called cycloalkanes Cycloalkanes – similar to alkanes – are saturated; all C-C bonds are single bonds They have the general molecular formula of C n H 2n (two less hydrogen then open chain alkanes)

Naming Cycloalkanes 1) Determine the name of the alkane with the same number of carbon atoms in the ring and add the prefix cyclo- Cyclopentane, cyclohexane etc. 2) If there is only one substituent, place the name of the substituent before the name of the cycloalkane (methylcyclohexane, chlorocyclohexane) 3) If there are more then one substituent, use numbers to give the lowest possible position number (1-ethyl-2-methyl cyclohexane) (1-ethyl-2-methyl cyclohexane)

Cis-trans Isomerism in Cycloalkanes Cis-trans isomerism is a type of geometric isomerism (stereoisomers) This isomerism is due to lack of free rotation around the bonds (energy barrier) – substituents are either above or below the ring They have different physical properties

Reactions of Alkanes: 1) Combustion Oxidation (burning) of hydrocarbons in the presence of excess oxygen is called combustion The reaction produces CO 2, water and releases large amounts of heat Fuels are hydrocarbons (methane, natural gas, gas used for vehicles, petroleum etc.) CH 4 + 2O 2  CO 2 + 2H 2 O + heat energy Some organisms use this reaction to produce energy

2) Halogenation Reaction of an alkane with a halogen (chlorine or bromine usually) to produce an alkyl halide (or haloalkane) and a hydrogen halide This reaction occurs through substitution mechanism This reaction converts unreactive alkanes to reactive alkyl halides, which are starting materials for synthesis of many organic compounds, including pharmaceuticals This reaction requires heat and/or light: CH 4 (g) + Cl 2 (g) CH 3 Cl(g) + HCl(g) (Multiple halogenations can also occur if the reaction is allowed to continue)

Unsaturated Hydrocarbons Unsaturated hydrocarbons contain at least one carbon- carbon double bond or triple bond Alkenes (double bond), alkynes (triple bond) and aromatic compounds (double bonds in a ring) are all groups of unsaturated hydrocarbons Many of the medically important molecules are characterized by their saturation/unsaturation; for example fatty acids can be either saturated or unsaturated

Alkenes and Alkynes Unsaturated hydrocarbons Alkenes: at least one double bond Alkynes: at least one triple bond Carbons in an alkane are bonded to 3 other atoms (hydrogen or carbon) – tetrahedral Carbons of the double bond are bonded to two other atoms – 120 degree – planar Carbons making up the triple bond is connected to only one other atom – 180 – linear Like alkanes, alkenes and alkynes are nonpolar, not soluble in water, soluble in nonpolar solvents – they have all similar melting and boiling points

Nomenclature of Alkenes and Alkynes IUPAC Rules: 1) Name the longest continuous carbon chain (alkane parent compound) containing the double or triple bond 2) Replace the –ane ending of the name with –ene for alkene and –yne for alkyne 3) Number the chain such that you give the lowest possible number for the first of the two carbons containing the double bond or triple bond 4) Determine the number and name of the substituents (the groups bonded to the parent chain); double and triple bond carbons get lower numbers compared to alkyl and halogen substituents 5) Alkenes with more than one double bond as alkadienes, alkatrienes etc. 3,4-Diethyl-1-hexene 3-Chloro-1,4-pentadiene

Cis-trans (Geometric) Isomers Rotation around the carbon-carbon double bond is restricted (requires very high energy – not possible at room temperature) This is due to pi-bonding and shape of orbitals

Cis versus Trans Fatty Acid Most unsaturated fatty acids in the body are cis isomers, Trans isomers (found in margarines) are harmful for health

Reactions of Alkenes and Alkynes The most important reaction of alkenes is addition reactions to the carbon-carbon double bond 1) Hydrogenation 2) Halogenation 3) Hydration 4) Hydrohalogenation

1) Hydrogenation Addition of a molecule of hydrogen (H 2 ) to the double bond to give an alkane Platinum, palladium or nickel are required as catalyst – heat and pressure also might be required Alkynes are hydrogenated in a similar way to alkenes (2 molecules of hydrogen molecule is needed to convert an alkyne to an alkane) Hydrogenation is used in food industry to convert vegetable oils (liquid) to margarine (solid) - unsaturated oils are converted into saturated fat

2) Halogenation: Addition of X 2 Chlorine (Cl 2 ) or bromine (Br 2 ) can be added to a double bond – proceeds easily, no need for catalyst Bromine test: Red color of bromine in solution is lost if bromine is added to the double bond Alkynes react the same way as alkenes

3) Hydration: Addition of H 2 O A water molecule can be added to an alkene double bond in the presence of trace acid hydrogen (-H) adds to one carbon, -OH adds to the other carbon The product of hydration reaction is an alcohol When an unsymmetrical alkene is hydrated, there are more than one product possible; how to know the major product? Markovnikov’s Rule: Among the two carbon atoms making the double bond, the carbon with more hydrogen atoms attached prefers receiving the hydrogen and the other carbon gets –OH Hydration is an important type of reaction in biological systems

4) Hydrohalogenation: Addition of HX Hydrogen halides (HBr, HCl, HI) can also be added to an alkene; the product is an alkyl halide This reaction also follows Markovnikov’s rule: Hydrogen adds to the double bond carbon with more hydrogen, halide adds to the double bond carbon with less hydrogen