Hydrocarbons. Organic structures that contain only carbon and hydrogen

Transcription

1 ydrocarbons Organic structures that contain only carbon and hydrogen Saturated a compound is termed saturated if it has the maximum hybridization (sp 3 ) at each carbon Therefore: no double or triple bonds A saturated carbon species is termed an ALKANE C 3 -C 3 ethane The compound has a root name indicating the number of carbons and the ane suffix

2 An ALKENE has a carbon-carbon double bond All three structures represent ethylene (or ethene) An ALKYNE has a carbon-carbon triple bond All three structures represent acetylene (or ethyne)

3 Straight Chain Alkanes The alkanes are named according to the number of carbon atoms in the chain Ends with an ane suffix Root name # of carbons (n) -(C 2 ) n - Meth- 1 Eth- 2 Prop- 3 But- 4 Pent- 5 ex- 6 ept- 7 Oct- 8 Non- 9 Dec- 10 All alkanes have the empirical formula C n (2n+2)

4 Origin of Naming for Alkanes C 1 through C 4 are result of common names for carbon chains, C 5 through C 10 are named due to the Greek word for their root (an 8 sided circle for example is an octagon OCT represents 8) Meth - means wine or spirit in Greek, yl means wood or matter in Greek Therefore methyl alcohol (which has one carbon) means a spirit from wood Methanol is obtained from distillation of wood (sometimes called wood alcohol) MET is thus kept for a 1 carbon chain, yl is kept to mean a carbon group and is used for any carbon substituent (methyl, ethyl, propyl, etc.) ET root comes from Greek word ether (to shine) Shine sky colorless liquid Ether (also called diethyl ether) is a colorless liquid and it has two 2-carbon chains a two carbon chain is ET

5 PROP common name is a result of the three carbon chain acid called propionic acid Protos (Greek for first), pion (Greek for fat) Propionic acid thus literally means first fat 1 carbon acid is formic acid (from ants) 2 carbon acid is acetic acid (from vinegar) Both formic acid and acetic acid are soluble in water due to the low carbon content, Propionic acid is thus the smallest acid chain that is not soluble in water but soluble in organic solvents (thus first fat fatty acids are long chain carboxylic acids) BUT comes from the common name for a 4 carbon carboxylic acid (butyric acid) Butyric acid is the cause for the smell in rancid butter (where BUT comes from the word for butter)

6 ofmann s attempt for Systematic ydrocarbon Nomenclature (1866) Attempted to use a systematic name by naming all possible structures with 4 carbons Quartane C 4 10 Quartyl C 4 9 Quartene C 4 8 Quartenyl C 4 7 Quartine C 4 6 Quartinyl C 4 5 Quartone C 4 4 Quartonyl C 4 3 Quartune C 4 2 Quartunyl C 4 1 Wanted to use Quart from the Latin for 4 this method was not embraced and BUT has remained

7 IUPAC Nomenclature Procedure for naming carbon chains containing branches or substituents (non-straight chain) 1) Find the longest continuous carbon chain in the structure -this determines the root name 2) Any carbon not on this continuous chain is a substituent (appendage) 3) Number the main chain starting from the end closest to the first substituent 4) The substituents are still named according to the number of carbons (the suffix for a substituent is yl instead of ane) -C 3 methyl -C 2 C 3 ethyl 5) Place all substituent names before the root name in alphabetical order

8 6) The substituent must be numbered to indicate the point of attachment to the main chain 7) Group multiple substituents of the same kind together and label di-, tri-, etc. 8) When alphabetizing, the prefixes di-, tri, n-, t- are ignored (the only prefix used for alphabetizing is iso-, explained in common names) 9) With a ring compound the number of carbons in the ring determines the root name with a cyclo- prefix 10) alogens are named as substituents with an o suffix e.g. fluoro-, chloro-, bromo- or iodo-

9 Common Names Many alkyl substituents have common names Consider propyl There are two ways an alkyl appendage with three carbons can be attached Any straight chain appendage has the n- prefix (for normal) C 3 C 2 C 2 - n-propyl This distinguishes the straight chain compound from the other isomer Isopropyl (1-methylethyl) using IUPAC Use iso prefix (short for isomer)

10 With larger alkyl substituents, the more possibilities for isomers exist Consider butyl 3 C 2 C 2 C 2 C n-butyl 3 C 3 C C C 2 isobutyl 3 C 3 C 2 C C secbutyl (s-butyl) 3 C C 3 C C 3 tertbutyl (t-butyl)

11 The sec- and tert- prefixes for common names are based upon degree of substitution A carbon bonded to three other carbons is called a tertiary carbon C 3 3 C tertiary carbon (3 ) C C 3 e.g. tertbutyl A carbon bonded to two other carbons is called a secondary carbon secondary carbon (2 ) 3 C 3 C 2 C C e.g. secbutyl A carbon bonded to one other carbon is a primary carbon 3 C 3 C C C 2 primary carbon (1 ) e.g. both n-butyl and isobutyl To name substituents, only consider the bonding pattern of the carbon directly bond to the main chain, and then consider how many other carbons are bonded to that carbon to obtain tert- or sec- names

12 Complex Alkyl Groups As the alkyl substituents become more complicated (e.g. more branching) the same IUPAC rules are followed and the name for the whole appendage is placed in parenthesis The root is the cyclooctane ring (usually the ring is used as a root although if the number of carbons in the substituent become larger then the ring could be named as a substituent) ethyl substituent 1,1,3-trimethylbutyl substituent (with substituents need to count from the carbon at the attachment to root and find longest chain) After alphabetizing: 1-ethyl-3-(1,1,3-trimethylbutyl)cyclooctane

13 Attractive Forces in Alkanes - Type of electron correlation between molecules determine the physical properties Coulombic attraction dipole-dipole van der Waals forces (London dispersion)

14 Conformational Analysis of Alkanes - Physical properties of molecules are determined by intermolecular forces (forces between molecules) - The internal structure of a given molecule can affect the energy due to sterics (intramolecular interactions) Conformer: different arrangements in space resulting from the rotation of bonds (bonds are not broken when interconverting between conformers)

15 Consider Methane No conformers possible; methane has a given energy value that does not change (any rotation about the equivalent C- σ bonds yields the same structure in three-dimensions) *this is not the case with any higher hydrocarbon homologue

16 Conformational Analysis of Ethane Structures have different energy due to different arrangements of space (hydrogens have different spatial arrangements in different conformers)

17 Newman Projections - Convenient way to view conformational analysis

18 To Draw Newman Projections 1) Determine which bond is being considered 2) Determine which atom is front atom of bond being considered 3) The substituents attached to the front atom are drawn to a point, the substituents attached to the back atom are drawn to a circle 4) The relative angles and orientation of the substituents are maintained

19 Newman projections of ethane conformations Newman projections demonstrate energetic and spatial interactions of conformers

20 Eclipsed conformations are higher in energy One cause is the sterics As the substituents that are eclipsed become larger, the energy of the conformer raises Consider the space filling area of atoms

21 Conformational Energy Diagram for Propane

22 Different Types of Interactions Arise with Larger Carbon Structures Consider n-butane viewing down the C 2 -C 3 carbon-carbon bond

23 Rings (Cycloalkanes) Due to the ring the σ bonds cannot rotate 360 as in alkanes Do not have the same conformational analysis as with other alkanes Therefore rings adopt a certain preferred geometry

24 Rings Strain for Simple Cycloalkanes Ring size cycloalkane Total ring strain (Kcal/mol) Ring strain per C 2 (Kcal/mol) 3 cyclopropane cyclobutane cyclopentane cyclohexane cycloheptane cyclooctane Small rings have large strain Cyclohexane has the least amount of strain

25 Conformation of Cyclopropane All three carbon atoms must be coplanar This geometry causes strain due to both small bond angles and torsional strain

26 Conformation of Cyclobutane Structure if constrained to plane actual structure Cyclobutane adopts a puckered conformation in order to lower torsional strain Still have high bond angle strain

27 Conformation of Cyclopentane The ring forms a preferred geometry to lower torsional strain The conformation is called the envelope due to its similarity to a mailing envelope

28 Conformation of Cyclohexane Cyclohexane has the least amount of ring strain The reason is the ability of the ring to form a stable conformation Planar cyclohexane (120 <C-C-C, All hydrogens eclipsed) Chair cyclohexane (nearly tetrahedral <C-C-C, no hydrogens eclipsed)

29 Names for Various Conformers of Cyclohexane Remove hydrogens Chair conformation Twist-boat conformation Boat conformation

30 Newman Projection for Chair Conformation The chair conformation has a low torsional strain as seen in a Newman projection Nearly perfect staggered alignment Still have some gauche interactions, but energy is low for this conformation

31 Chair-Chair Interconversion with Cyclohexane Key point there are two distinct chair conformations for a cyclohexane that can interconvert

32 6-Membered Rings are Observed Frequently in Biological Molecules

33 The 12 substituents in a chair (12 hydrogens for cyclohexane) occur in two distinct types of positions Pole (axial) equator In flat conformation, all hydrogens are identical In chair conformation, 2 sterically different positions occur

34 The Axial and Equatorial Positions have Different Spatial Requirements There are two chair conformations, a substituent moves from equatorial to axial in a chair-chair interconversion Y is equatorial Y is axial Bigger Y substituent has more steric interactions in an axial position than equatorial The chair conformation which has the Y group equatorial is therefore more stable

35 A substituent would prefer to be in an equatorial position If there are two substituents they will compete for the equatorial position An ethyl group is bigger than a methyl so therefore this compound would prefer the left conformation If both substituents can be in the equatorial position than this conformer will be heavily favored If there are more substituents, need to compare the cumulative sterics for all substituents to predict which chair is more stable

36 Don t confuse Equatorial/Axial with Cis/Trans A cis/trans ring junction refers to whether both substituents are on the same side or opposite sides of a ring