Unsaturated Hydrocarbons Chem 1152: Ch. 12 Unsaturated Hydrocarbons

Types of Hydrocarbons Saturated: All C—C bonds are single bonds Unsaturated: Hydrocarbons with one or more multiple bonds The term unsaturated is used because C could bind more H (become saturated) if no multiple bonds existed.

Rules for Naming Alkenes Name the longest chain that contains the double bond or double bonds. The name of the chain will end in –ene. Number this longest chain so the C=C bond or bonds has/have the lowest number. The first C of the C=C bond (for C=C bond to have lowest number) identifies the positional location of the double bond. Name the attached functional groups. Combine the names of the attached groups and longest chain, the same as you would with alkanes. For multiple double bonds, indicate the locations of all multiple bonds, use numeric prefixes indicating number of double bonds (-diene, -triene). Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

Naming Alkenes Step 1: Name the longest chain that contains the C=C bond. Use the IUPAC root and the –ene ending. Step 2: Number the longest chain so the C=C bond gets the lowest number possible. Step 3: Locate the C=C bond with the lower-numbered carbon. Examples: 1 2 3 4 CH3-CH=CH-CH3 2-butene 6 5 4 3 2 1 CH3-CH2-CH2-CH=CH-CH3 2-hexene

Naming Alkenes (continued) Step 4: Locate and name attached groups. Step 5: Combine all the names.

3-bromo-2-methyl-1-propene 5-ethyl-3-methyl-2-octene Naming Alkenes 2-pentene 4-methylcyclohexene 2-ethyl-1-hexene 3-bromo-2-methyl-1-propene 5-ethyl-3-methyl-2-octene Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

Alkene Examples

The Geometry of Alkenes In C=C bonds, sp2 hybrid orbitals are formed by the carbon atoms, with one electron left in a 2p orbital. A representation of sp2 hybridization of carbon. During hybridization, two of the 2p orbitals mix with the single 2s orbital to produce three sp2 hybrid orbitals. One 2p orbital is not hybridized and remains unchanged.

The Geometry of Alkenes (continued) One bond (sigma, σ) is formed by overlap of two sp2 hybrids. The second bond (pi, π) is formed by connecting the unhybridized p orbitals. Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

The Geometry of Alkenes (continued) The planar geometry of the sp2 hybrid orbitals and the ability of the 2p electron to form a “pi bond” bridge locks the C=C bond firmly in place. Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

The Geometry of Alkenes (continued) Because there is no free rotation about the C=C bond, geometric isomerism is possible. cis- isomers have two similar or identical groups on the same side of the double bond. trans- isomers have two similar or identical groups on opposite sides of the double bond. Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

The Geometry of Alkenes (continued) Geometric isomers have different physical properties. Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

Geometric (cis-trans) Isomerism in Alkenes By definition, must have 2 different groups attached to each double bonded carbon. Which of the following can exhibit cis-trans isomerism?   Cl-CH=CH-Cl CH2=CHCH3 Cl-CH=CHCH3

Properties of Alkanes and Alkenes Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

Types of Alkene Reactions C=C The double-bond in alkenes make them chemically reactive Four major types of alkene reactions Addition Elimination Substitution Rearrangement

A + B  C Addition Reactions Two reactants add together to form a single new product that includes all original atoms. Double bond becomes a single bond.

Addition Reactions Halogenation: Addition of a Halide (fluorine, chlorine, bromine, iodine). Hydrogenation: Metal (Pt, Pd or Ni) catalyzed addition of hydrogen atoms to C=C bond. Pt

Addition Reactions Acid Rxn: (HCl, HBr, etc.) The major product of this rxn is 2-bromopropane, due to: Markovnikov’s rule: When H-X reacts with alkene, H goes to C that already has the most H.

Addition Reactions Hydration: Water may react with alkene in presence of acid catalyst Rxn also follows Markovnikov’s rule This method can be used for large scale production of alcohols

Addition Reactions Polymerization: Addition rxn where identical molecules join together to form long chains. This rxn can produce very long, inert alkane chains, useful in manufacture of food storage containers.

Addition Reactions Copolymerization: Addition rxn with 2 different monomers. Note: --CH=CH2 is known as a vinyl group

Elimination Reactions A  B + C Single reactant becomes multiple products. Single bond becomes a double bond.

Substitution Reactions 2 reactants exchange parts, create 2 new products.

Rearrangement Reactions Single reactant reorganized into isomer.

Geometry of Alkynes Copolymerization: Addition rxn with 2 different monomers. Insoluble in water Less dense than water Low MP, BP Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

Aromatics 6 C ring structures with alternating double bonds (benzene). Everything that is not aromatic, is aliphatic (alkanes, alkenes, alkynes). Early problem chemists found was that benzene was not reactive (recall that alkenes are reactive due to double bond). Kekule proposed that double bonds alternated between 2 equivalent structures electrons move around a conjugated pi bond system of rings Stabilizes structure and makes it less reactive.

Naming Benzene Derivatives For single replacement (H some FG), cmpd named as a benzene derivative.

Naming Benzene Derivatives (continued) Some common names are IUPAC-accepted and used preferentially. Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

Naming Benzene Derivatives Some benzene derivatives better known by common names accepted by IUPAC. aniline  common aminobenzene  IUPAC The benzene ring can also be an attached substituent group.

Naming Benzene Derivatives With only 2 groups on benzene ring, can use o, m, p (ortho, meta, para) nomenclature.

Naming Benzene Derivatives When 2 or more groups are attached to benzene ring, positions can be indicated by numbering ring C atoms to obtain lowest possible numbers.

Naming Alkynes Name of the compound ends in -yne. The longest chain chosen for the root name must include both carbon atoms of the triple bond. The root chain must be numbered from the end nearest a triple bond carbon atom. If the triple bond is in the center of the chain, the nearest substituent rule is used to determine the end where numbering starts. The smaller of the two numbers designating the carbon atoms of the triple bond is used as the triple bond locator. If several multiple bonds are present, each must be assigned a locator number. Double bonds precede triple bonds in the IUPAC name, but the chain is numbered from the end nearest a multiple bond, regardless of its nature. The name will then have multiplier prefix (e.g., diyne, triyne, etc.) Because the triple bond is linear, it can only be accommodated in rings larger than ten carbons. In simple cycloalkynes the triple bond carbons are assigned ring locations #1 and #2. Which of the two is #1 may be determined by the nearest substituent rule. Substituent groups containing triple bonds are:             HC≡C–   Ethynyl group

Examples of Alkynes

Examples of Alkynes

Examples of Alkynes