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Understanding Hydrocarbons: Alkanes, Alkenes, and Alkynes in Organic Chemistry

This comprehensive guide explores hydrocarbons, focusing on alkanes, alkenes, and alkynes in organic chemistry. Hydrocarbons are organic compounds made exclusively of carbon and hydrogen, with alkanes characterized by single bonds (CnH2n+2) and being saturated with hydrogen. Alkenes have at least one double bond (CnH2n), while alkynes contain triple bonds (CnH2n-2). The naming conventions for these compounds, including prefixes and structural formulas, are explained with examples, substitutions, and practice problems to facilitate learning.

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Understanding Hydrocarbons: Alkanes, Alkenes, and Alkynes in Organic Chemistry

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  1. Honors ChemistryHydrocarbons

  2. Organic Prefixes • The number of carbons in a chain is indicated with a prefix.

  3. Hydrocarbons - Alkanes • A hydrocarbon is an organic molecule consisting of only carbon and hydrogen. • Hydrocarbons are used as fuels. • An alkane is a hydrocarbon that has only single bonds. • Also referred to as a saturated hydrocarbon (saturated with hydrogens) • General formula: CnH2n+2 • Examples: • Propane(C3H8) • Octane (C8H18)

  4. Practice – Straight Chain Alkanes • Name the following alkanes; • C2H6 • C7H16 • CH4 • What is the formula for: • Butane • Nonane • Hexane

  5. Branched Alkanes • Hydrocarbons are not limited to straight lines. • We can substitute “alkyl” groups onto a larger chain. • In this example, we have attached a “methyl” (one carbon) group to butane.

  6. Branched Alkanes • In order to name this alkane, we need to specify which carbon the substitution happens. • So, we number the carbons, starting with the end of the molecule closest to where the substitution happens, and creating the longest chain possible. • The methyl group is on carbon #2. • The name of this molecule is2-methylbutane.

  7. Multiple Substitutions • If there are multiple substitutions, name the substitutions alphabetically. • If you have the same type of substitution, include both numbers and a Greek prefix before the alkyl group. • 1) Find the longest chain. • 2) Number from the end of the first substitution. • 3) Arrange the substitutions alphabetically and name the compound as one word with hyphens separating the numbers. • 4) If there is a tie, lowest total numbers win.

  8. Alkenes • An alkene is formed when a double bond is present in the carbon chain. • The suffix “-ene” is used to denote the name. • Have the general formula CnH2n. • Example: propene (C3H6)

  9. Alkenes • If the double bond occurs on the first carbon, it is said to be “normal” or “n”. Using the number “1” is also sufficient. • Example: n-butene, or 1-butene

  10. Alkenes • However, if the double bond occurs beyond the first carbon, the name reflects its location. • In an alkene (or later, alkyne), the longest chain MUST include the double bond, even if this results in a shorter chain. • The numbering of the carbons ALWAYS is closest to a double/triple bond, regardless of substituent placement. 2-pentene

  11. Practice • Give the structural formulas for the following: • 3-hexene • 2-methyl-1-butene • Give the molecular formulas for: • butene • pentene

  12. Multiple Double Bonds • If there are multiple double bonds in a structure, indicate this in the ending of the name as a dieneor a triene, etc. • Example: • 5-methyl-2,4-heptadiene

  13. Alkynes • Alkynes are hydrocarbons that contain triple bonds. • They operate under the same naming conditions as alkenes. • They have the generic formula CnH2n-2 • Example: ethyne (C2H2)

  14. Quick Check • Give the molecular formula for the following: • Butyne • Octyne • Propyne • Give the name for the following: • C4H6 • C10H18 • C6 H10

  15. IUPAC Alkyne Practice • The same exact rules for alkenes apply for alkynes. • Name this alkyne:

  16. Isomers • An isomer are organic compounds that have the same molecular formula but different structural formulas. • Example: pentane (C5H12) “n-Pentane” “Isopentane” “Neopentane”

  17. Isomers • How many possible isomers are possible for butane? Draw them.

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