Alkanes from Carbon and Hydrogen

Transcription

1 hapter 2: Alkanes

2 Alkanes from arbon and ydrogen Alkanes are carbon compounds that contain only single bonds. The simplest alkanes are hydrocarbons compounds that contain only carbon and hydrogen. ydrocarbons are used mainly as fuels, solvents and lubricants: # of carbons boiling point range Use < >400 fuel (gasses such as methane, propane, butane) solvents (petroleum ether) solvents (ligroin) fuel (gasoline) fuel (kerosene) fuel (heating oil) lubricating oil, asphalt

3 ydrocarbons mula Prefix Suffix Name Structure 4 meth- methane 2 6 eth- ethane prop- propane 4 10 but- butane 5 12 pent- pentane 6 14 hex- hexane 7 16 hept- heptane 8 18 oct- octane 9 20 non- nonane dec- decane

4 ydrocarbons mula Prefix Suffix Name Structure 4 meth- methane 2 6 eth- ethane 3 8 prop- propane but- butane but- butane? 3 3 3

5 ydrocarbons mula Prefix Suffix Name Structure 4 meth- methane 2 6 eth- ethane 3 8 prop- propane but- butane but- iso-butane 3 3 3

6 ydrocarbons mula Prefix Suffix Name Structure 4 10 but- butane but- iso-butane pent- pentane pent- iso-pentane pent- 3 iso-pentane??? 3

7 ydrocarbons mula Prefix Suffix Name Structure 4 10 but- butane but- iso-butane pent- pentane pent- iso-pentane pent- neo-pentane 3 3

8 IUPA System of Naming Organic ompounds 1. ind the longest carbon chain 2. Name all the atoms (other than ) not in the longest chain as groups 3. Number groups from the end of the chain closest to the first group 4. List multiple groups in alphabetical der, Longest chain: pentane Groups: none IUPA Name pentane Longest chain: butane Groups: one methyl 2-methylbutane not: 3-methylbutane not: Longest chain: propane Groups: two methyls 2,2-dimethylpropane 3

9 IUPA System of Naming Organic ompounds mula Prefix Suffix IUPA Name Structure 4 10 but- butane but- iso-butane pent- pentane pent- 2-methyl butane pent- 2,2-dimethylpropane 3

10 Naming Branches as Groups Group mula Prefix Suffix Name Abbreviation Structure 3 meth- -yl methyl 2 5 eth- -yl ethyl Et 3 7 prop- -yl propyl Pr iso-prop- -yl iso-propyl i Pr but- -yl butyl Bu 3

11 Naming Branches as Groups Group mula Name Structure Group mula Name Structure 4 9 iso-butyl fluo 4 9 sec-butyl l chlo l 4 9 tert-butyl Br bromo Br 5 11 neo-pentyl I iodo I

12 Structural Isomers ow many structural isomers compounds with the same chemical fmula but different connectivity are there with the fmula 7 16?

13 Structural Isomers ow many structural isomers compounds with the same chemical fmula but different connectivity are there with the fmula 7 16?

14 thyl and Ethyl ompounds Group mula Name Abbreviation Structure 3 methyl 5 2 ethyl Et 3 methane 5 2 ethane 3 O methyl alcohol (methanol) 5 2 O ethyl alcohol (ethanol) 3 N 2 methylamine (aminomethane) 5 2 N 2 ethylamine (aminoethane) 3 methyl fluide (fluomethane) 5 2 ethyl fluide (fluoethane) 3 l methyl chlide (chlomethane) 5 2 l ethyl chlide (chloethane) 3 Br methyl bromide (bromomethane) 5 2 Br ethyl bromide (bromoethane) 3 I methyl iodide (iodomethane) 5 2 I ethyl iodide (iodoethane) 3 N methyl cyanide (cyanomethane, acetonitile) 5 2 N ethyl cyanide (cyanoethane, proprionitile) 3 S methanethiol (methyl mercaptan) 5 2 S ethanethiol (ethyl mercaptan)

15 thyl and Ethyl ompounds Group mula Name Abbreviation Structure 3 methyl 5 2 ethyl Et 3 methane ethane 3 methyl cation 3 2 ethyl cation 3 methyl radical ethyl radical 3 methyl anion ethyl anion

16 Propyl ompounds Group mula Name Abbreviation Structure 7 3 prop- Pr iso-prop- i Pr O propyl alcohol (1-propanol) O 2 3 isopropyl alcohol (isopropanol; 2-propanol) N 2 propyl amine (1-aminopropane) 2 N 1 isopropyl amine (2-aminopropane) propyl fluide (1-fluopropane) isopropyl fluide (2-fluopropane) l propyl chlide (1-chlopropane) l isopropyl chlide (2-chlopropane) Br propyl bromide (1-bromopropane) Br isopropyl bromide (2-bromopropane) I propyl iodide (1-iodopropane) I isopropyl iodide (2-iodopropane)

17 Structural Isomers of 7 16 What if we attatch the two ends together? heptane cycloheptane 7 14 ycloheptane contains two fewer hydrogens than heptane. They are not structural isomers.

18 ycloalkanes ompound Name Structure Group Name Structure cyclopropane cyclopropyl cyclobutane cyclobutyl cyclopentane cyclopentyl cyclohexane cyclohexyl cycloheptane cycloheptyl cyclooctane cyclooctyl

19 Naming ycloalkanes Unlike "floppy" chains, rings have an additional consideration f multiple substitutions: "flat" 1,2-dimethylcyclopropane "3-D" 1,2-dimethylcyclopropane "3-D" 1,2-dimethylcyclopropane These are different compounds! They cannot be interconvertied by bond rotation. cis-1,2-dimethylcyclopropane trans-1,2-dimethylcyclopropane cis substituents on same side (of the ring) trans substituents on opposite sides (of the ring) The above compounds are not structural isomers. Their connectivity is identical. They are stereoisomers compounds that differ in how substituents are arranged in space

20 Naming ycloalkanes ow can we represent cis/trans isomers in a flat drawing? cis-1,2-dimethylcyclopropane trans-1,2-dimethylcyclopropane A solid wedge means the bond is pointing "up" and out of the page A dashed wedge means that the bond is pointing "down" and into the page

21 Naming ycloalkanes Don't let the wedges and dashes confuse you: cis-1,2-dimethylcyclopropane cis-1,2-dimethylcyclopropane These are the same compound! lip over 180

22 Nuclear Magnetic Resonance (NMR) Spectroscopy This technique allows us to observe the nuclei of certain atoms, such as hydrogen and carbon. While we will learn about NMR much me extensively, f now: NMR will give a signal f each "different" atom in a molecule. What is a "different" atom? One that has a unique environment/connectivity. O 4 O O # of carbon NMR signals: 1 1 1

23 Nuclear Magnetic Resonance (NMR) Spectroscopy This technique allows us to observe the nuclei of certain atoms, such as hydrogen and carbon. While we will learn about NMR much me extensively, f now: NMR will give a signal f each "different" atom in a molecule. What is a "different" atom? One that has a unique environment/connectivity. 4 O O # of carbon NMR signals: O 3 3 # of carbon NMR signals: 1 1

24 3-Dimensional Structure/VSEPR They X A dashed wedge means: the substituent is behind the plane of the paper Atoms place substituents as far appart as possible: 109 A solid wedge means: the substituent is in front of the plane of the paper X X A X X A X X A = any atom X X = any substituent (atom lone pair) N O B tetrahedral electron-domain geometries

25 3-Dimensional Structure of Ethane each carbon is tetrahedral "flat" ethane wedge/dash depiction of ethane "3-D" ethane

26 3-Dimensional Structure of Ethane If we look down the bond axis, we get a different perspective: We can use a Newman Projection to depict this view: The large circle is a stylized representation of the back atom ront atom appears at vertex here Newman Projection But there is free rotation around single (σ) bonds: The "staggered" confmation these hydrogens are in the same plane these hydrogens are in the same plane The "eclipsed" confmation

27 3-Dimensional Structure of Ethane The "staggered" confmation these hydrogens are in the same plane these hydrogens are in the same plane The "eclipsed" confmation The staggered confmation is the lowest energy confmation The elclpsed confmation is the highest energy confnation The energy minimum The energy maximum Molecules that differ by only rotations around single bonds are confmational Isomers.

28 3-Dimensional Structure of Butane energy Ref = 0 A E 1 E G staggered confmation gauche confmation Δ E = kcal/mol

29 3-Dimensional Structure - ybridization Place substituents as far appart as possible: N 3 N Molecule is linear B 3 B 120 Molecule is trigonal planar 2s 2p x 2p y 2p z 109 2nd row valence bitals 4 Molecule is tetrahedral ow can these molecular shapes arise from bonding between the available bitals?

30 ybridization N 3 N Molecule is linear 2s 2p x 2p y 2p z 2nd row valence bitals this side gets bigger and 50% me spherical this side gets smaller and 50% me spherical 2p x mixing and 2s 2 unhybridized bitals 2 sp-hybridized bitals Only "X"-directionality; 50% s-like and 50% p-like 3 N 3 N 3 N sp-bitals on the central carbon atom the σ-bond fmed by sp-bital overlap π-bonding in the triple bond

31 ybridization 120 B 3 B Molecule is trigonal planar 2s 2p x 2p y 2p z 2nd row valence bitals this side gets bigger and 33% me spherical this side gets smaller and 33% me spherical 2p x mixing and and 2s 2p y 3 unhybridized bitals 3 sp-hybridized bitals Only "X" and "Y"-directionality; 33% s-like and 67% p-like B B All 3 sp 2 -bitals on the bon atom The σ-bonds between bon and fluine

32 ybridization s Molecule is tetrahedral 2px 2py 2pz 2nd row valence bitals mixing 2s 2px 2py 2pz All 4 unhybridized bitals All 4 sp3-bitals on the carbon atom The σ-bonds between carbon and hydrogen Each bital has "X", "Y" and "Z"-directionality and are 25% s-like and 75% p-like a "hand drawn" sp3 bital a "real" sp3 bital