Reactions of Hydrocarbons Alkanes, alkenes, alkynes and aromatics

Types of Hydrocarbons Hydrocarbons contain C and H There are 4 families, alkanes only have C-C single bonds, alkenes have at least one C=C and alkynes have a least one CΞC bond, aromatics are like cyclohexane with 3 C=C double bonds

Names of Alkanes The names all end in “ane”, the prefix depends on how many carbons are in the alkane

Cycloalkanes In a cycloalkane a group of C atoms form a ring Consider Cyclobutane C4H8 Complete Structural Formula Geometric Formula Condensed Formula 3-dimensional structure

Nomenclature for Alkenes Alkenes have one or more C=C Step 1: Name the longest chain with the C=C in it Step 2: Number the longest chain so that the carbons in the C=C have the lowest numbers Step 3: Locate the lowest numbered C in the C=C Step 4: Locate and name attached groups Step 5: Combine the names

Naming Alkenes 2 carbons prefix eth- 1 C=C suffix –ene Name = ethene 3 carbons prefix prop- 1 C=C suffix –ene Name = propene 4 carbons prefix but- 1 C=C suffix –ene Name = butene But where is the C=C? 1 butene (or but-1-ene) 2 butene (or but-2-ene)

Naming Cycloalkenes 6 carbons prefix hex- 1 C=C suffix –ene Name = hexene 6 carbons in a ring cyclohexene 5 carbons prefix hex- 1 C=C suffix –ene Name = pentene 5 carbons in a ring cyclopentene

Naming Alkynes 2 carbons prefix eth- 1 CΞC suffix –yne Name = ethyne 3 carbons prefix prop- 1 CΞC suffix –yne Name = propyne 4 carbons prefix but- 1 CΞC suffix –yne Name = butyne But where is the CΞC? 1 butyne (or but-1-yne) 2 butyne (or but-2-yne)

Aromatics Aromatics contain This is called benzene This is called toluene (methyl benzene)

Identifying the type of hydrocarbon Alkanes, alkenes, alkynes and aromatics have different chemical properties and we can identify what we have by performing 3 tests Reaction with O2 (combustion) Reaction with Br2 (Bromine test) Reaction with KMnO4 (Potassium Permanganate test)

Reaction with O2 (combustion) All hydrocarbons react with O2 to make CO2 (with some CO and C) and H2O CH4 + 2O2  CO2 + 2H2O (one CO2 for every C and one H2O for every 2H) If all hydrocarbons react how can this tell us if we have an alkane, alkene, alkyne or aromatic? Heavier hydrocarbons (more C) don’t evaporate as easily so less easy to combust Alkynes, Aromatics and cycloalkanes and cycloalkenes burn with sooty flames Ethyne (acetylene) aromatic methane cyclohexene

Reaction with Br2 (Bromine test) Add alkane or alkene Alkane Alkene Alkenes and alkynes quickly react with Br2 to make haloalkanes (colorless) Alkanes don’t react (except slowly in bright light) Benzene doesn’t react toluene will Br2 water

Reaction with KMnO4 (Potassium Permanganate test) In this test KMnO4 (purple) reacts with alkenes but not alkanes or aromatics Alkane or aromatic (negative test) Alkene (positive test) alkene diol

The Experiment Make models for ethene, propene, cyclobutene, cis-2-butene and ethyne and draw them In fume hood combust 5 drops of cyclohexane, cyclohexene, toluene and your unknown, record observations In the fume hood add 3-4 drops Br2 solution to 15 drops of cyclohexane, cyclohexene, toluene and your unknowns, record observations, then move to a window (direct light) To 5 drops of cyclohexane, cyclohexene, toluene and your unknown add 15 drops of 1% KMnO4 record your observations From your results identify your unknown as an alkane, or an alkene