Hydrocarbons Organic structures that contain only carbon and hydrogen Saturated – a compound is termed “saturated” if it has the maximum hybridization (sp3) at each carbon Therefore: no double or triple bonds A saturated carbon species is termed an ALKANE CH3-CH3 ethane The compound has a root name indicating the number of carbons and the –ane suffix 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) 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) H-(CH2)n-H Meth- 1 Eth- 2 Prop- 3 But- 4 Pent- 5 Hex- 6 Hept- 7 Oct- 8 Non- 9 Dec- 10 All alkanes have the empirical formula CnH(2n+2) Origin of Naming for Alkanes C1 through C4 are result of common names for carbon chains, C5 through C10 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) METH 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.) ETH 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 ETH 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) Hofmann’s attempt for Systematic Hydrocarbon Nomenclature (1866) Attempted to use a systematic name by naming all possible structures with 4 carbons Quartane C4H10 Quartyl C4H9 Quartene C4H8 Quartenyl C4H7 Quartine C4H6 Quartinyl C4H5 Quartone C4H4 Quartonyl C4H3 Quartune C4H2 Quartunyl C4H1 Wanted to use Quart from the Latin for 4 – this method was not embraced and BUT has remained 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) -CH3 methyl -CH2CH3 ethyl 5) Place all substituent names before the root name in alphabetical order 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) Halogens are named as substituents with an o suffix e.g. fluoro-, chloro-, bromo- or iodo- 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) CH3CH2CH2- n-propyl This distinguishes the straight chain compound from the other isomer Isopropyl (1-methylethyl) using IUPAC Use iso prefix (short for isomer) With larger alkyl substituents, the more possibilities for isomers exist Consider butyl H3CH2CH2CH2C n-butyl H3C CH CH2 isobutyl H3C H3C CH secbutyl H3CH2C (s-butyl) CH3 H3C C tertbutyl CH3 (t-butyl) 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 CH3 H3C C e.g. tertbutyl tertiary carbon CH3 (3˚) A carbon bonded to two other carbons is called a secondary carbon H3C secondary carbon CH e.g. secbutyl (2˚) H3CH2C A carbon bonded to one other carbon is a primary carbon H3C CH CH2 e.g. both n-butyl and isobutyl H3C primary carbon (1˚) 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 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 Attractive Forces in Alkanes - Type of electron correlation between molecules determine the physical properties Coulombic attraction dipole-dipole van der Waals forces (London dispersion) 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) Consider Methane H H H H No conformers possible; methane has a given energy value that does not change (any rotation about the equivalent C-H σ bonds yields the same structure in three-dimensions) *this is not the case with any higher hydrocarbon homologue Conformational Analysis of Ethane Structures have different energy due to different arrangements of space (hydrogens have different spatial arrangements in different conformers) Newman Projections - Convenient way to view conformational analysis 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 Newman projections of ethane conformations Newman projections demonstrate energetic and spatial interactions of conformers 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 Conformational Energy Diagram for Propane Different Types of Interactions Arise with Larger Carbon Structures Consider n-Butane viewing down the C2-C3 carbon-carbon bond 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 Rings Strain for Simple Cycloalkanes Ring size cycloalkane Total ring Ring strain strain per CH2 (Kcal/mol) (Kcal/mol) 3 cyclopropane 27.6 9.2 4 cyclobutane 26.4 6.6 5 cyclopentane 6.5 1.3 6 cyclohexane 0 0 7 cycloheptane 6.3 0.9 8 cyclooctane 9.6 1.2 Small rings have large strain Cyclohexane has the least amount of strain Conformation of Cyclopropane All three carbon atoms must be coplanar This geometry causes strain due to both small bond angles and torsional strain 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 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 Conformation of Cyclohexane Cyclohexane has the least amount of ring strain The reason is the ability of the ring to form a stable conformation H H 120˚ H H H H H H H H H H H H H H 111.4˚ H H H H H H H H Planar cyclohexane Chair cyclohexane (120˚ <C-C-C, (nearly tetrahedral <C-C-C, All hydrogens eclipsed) no hydrogens eclipsed) Names for Various Conformers of Cyclohexane H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H Remove hydrogens Chair conformation Twist-boat Boat conformation conformation 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 Chair-Chair Interconversion with Cyclohexane Key point – there are two distinct chair conformations for a cyclohexane that can interconvert 6-Membered Rings are Observed Frequently in Biological Molecules The 12 substituents in a chair (12 hydrogens for cyclohexane) occur in two distinct types of positions Pole (axial) H H H H H H H H H