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Chapter 3- Organic Compounds: Alkanes and Their Stereochemistry

Ashley  Piekarski,  Ph.D.  

Why am I learning this, Dr. P?

•  Alkanes  are  unreac7ve,  but  provide  useful  vehicle  to  introduce  important  ideas  about  organic  compounds  

•  Alkanes  will  be  used  to  discuss  basic  approaches  to  naming  organic  compounds  

•  We  will  take  an  ini7al  look  at  3-­‐D  aspects  of  molecules  

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Functional groups

•  Func%onal  group  •  collection of atoms at a site that have a

characteristic behavior in all molecules where it occurs

•  The group reacts in a typical way, generally independent of the rest of the molecule

•  For example, the double bonds in simple and complex alkenes react with bromine in the same way

Functional groups

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Functional groups with multiple carbon-carbon bonds

•  Alkenes  have  a  C-­‐C  double  bond  •  Alkynes  have  a  C-­‐C  triple  bond  •  Arenes  have  special  bonds  that  are  represented  as  alterna7ng  single  and  double  C-­‐C  bonds  in  a  six-­‐membered  ring  

Functional groups with multiple carbon-carbon bonds

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Functional groups with carbon singly bonded to an electronegative atom

Groups with a carbon-oxygen double bond (carbonyl groups)

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Summary of functional groups

Summary of functional groups

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Learning check

•  Iden7fy  the  func7onal  groups  in  benzocaine,  a  topical  pain  reliever.  

Learning check

•  Iden7fy  the  func7onal  groups  in  the  tetrapep7de:  Val-­‐Gly-­‐Ser-­‐Ala  

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Alkanes and Alkane Isomers

•  Alkanes  •  compounds with C-C single bonds and C-H

bonds only (no functional groups) •  connecting carbons can lead to large or small

molecules •  The formula for an alkane with no rings in it

must be CnH2n+2 •  Alkanes are saturated with hydrogen (no

more can be added) •  They are called aliphatic compounds

Alkane isomers

•  What  is  an  isomer?  •  compounds with the same molecular formula but

different structural formula •  The  molecular  formula  of  an  alkane  with  more  than  three  carbons  can  give  more  than  one  structure  

•  Alkanes  with  carbons  connected  to  no  more  than  2  other  carbons  are  straight-­‐chain  or  normal  alkanes  

•  Alkanes  with  one  or  more  carbons  connected  to  3  or  4  carbons  are  branched-­‐chain  alkanes  

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Alkane isomers

Constitutional isomers

•  Isomers  that  differ  in  how  their  atoms  are  arranged  in  chains  are  called  cons7tu7onal  isomers  

•  Compounds  other  than  alkanes  can  be  cons7tu7onal  isomers  of  one  another  

•  They  must  have  the  same  molecular  formula  to  be  isomers  

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Constitutional isomers

Condensed structures of alkanes

•  We  can  represent  an  alkane  in  a  brief  form  or  in  many  types  of  extended  form  

•  A  condensed  structure  does  not  show  bonds  but  lists  atoms,  such  as  •  CH3CH2CH2CH3 (butane) •  CH3(CH2)2CH3 (butane)

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Butane

Names of straight-chain alkanes

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Learning check

•  Draw  structures  for  the  five  possible  isomers  of  C6H14  

Learning check

•  Draw  two  isomeric  esters  with  the  formula  C5H10O2  

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Alkyl groups

•  Alkyl  group  •  remove one hydrogen from an alkane •  general abbreviation “R” for the rest of the

molecule •  Name: replace –ane ending of alkane with –yl

ending

Alkyl groups

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Types of alkyl groups

•  Classified  by  the  connec7on  site  •  a carbon at the end of a chain (primary alkyl

group) •  a carbon in the middle of a chain (secondary

alkyl group) •  a carbon with three carbons attached to it

(tertiary alkyl group)

Type of alkyl groups

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Terminology

•  The  terms  primary,  secondary,  ter7ary,  and  quaternary  are  rou7nely  used  in  organic  chemistry,  so  their  meanings  need  to  become  second  nature  

Terminology

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Learning check

•  Iden7fy  the  carbons  in  the  following  molecules  as  primary,  secondary,  ter7ary,  or  quaternary:  

Naming alkanes

•  Compounds  are  given  systema7c  names  by  process  that  uses:  

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Naming alkanes

•  Follow  specific  rules  (for  consistency!):  •  Find parent hydrocarbon chain •  Carbons in that main chain are numbered in

sequence •  Substituents are identified and numbered •  Write compound name in a single word •  Name a complex substituent as though it were

itself a compound

Parent hydrocarbon

•  Find  the  longest  con7nuous  chain  of  carbon  atoms  in  the  molecules,  and  use  the  name  of  that  chain  as  the  parent  name  

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Parent hydrocarbon

•  If  two  different  chains  of  equal  length  are  present,  choose  the  one  with  the  larger  number  of  branch  points  as  the  parent  

Number the atoms in main chain

•  Beginning  at  the  end  nearer  the  first  branch  point,  number  each  carbon  atom  in  the  parent  chain  

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Number the atoms in main chain

•  If  there  is  branching  an  equal  distance  away  from  both  ends  of  the  parent  chain,  begin  numbering  at  the  end  nearer  the  second  branch  point  

Identify and number the substituents

•  Assign  a  number  to  each  subs7tuent  to  locate  its  point  of  a_achment  to  the  parent  chain  

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Identify and number the substituents

•  If  there  are  two  subs7tuents  on  the  same  carbon,  give  both  the  same  number.    There  must  be  as  many  numbers  in  the  name  as  there  are  subs7tuents.  

Write the name as a single word

•  Use  hyphens  to  separate  the  different  prefixes,  and  use  commas  to  separate  number.  If  two  or  more  subs7tuents  are  present,  cite  them  in  alphabe7cal  order  (do  not  include  prefixes  in  this  rule).  

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Name a complex substituent as though it were a compound

Common names

Learn  these  common  names  

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Learning check

•  What  is  the  IUPAC  name  for  the  following  alkane?  

3-ethylhexane

2,3,5,5,7-pentamethyloctane

3-ethyl-4-methylhexane

Learning check

•  What  is  the  IUPAC  name  for  the  following  alkane?  

3-ethylhexane

2,3,5,5,7-pentamethyloctane

3-ethyl-4-methylhexane

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Learning check

•  What  is  the  structure  for  3-­‐ethylhexane?  

Properties of alkanes

•  Referred  to  as  paraffins  (low  affinity  compounds)  

•  They  will  burn  in  a  flame,  producing  carbon  dioxide,  water,  and  heat  •  What type of reaction is this called?

•  They  react  with  Cl2  in  the  presence  of  light  to  replaces  the  hydrogens  with  chlorines  

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Properties of alkanes

Physical properties

•  Boiling  points  and  mel7ng  points  increase  as  the  size  of  the  alkane  increases  

•  Dispersion  forces  increase  as  molecule  size  increases,  resul7ng  in  higher  mel7ng  and  boiling  points  

•  Branching  lowers  an  alkane’s  boiling  point  •  Why?

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Physical properties

Conformations of ethane

•  Stereochemistry  •  branch of chemistry concerned with the 3-

dimensional aspects of molecules •  the exact 3D structure is crucial to

determining its properties and biological activity

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Conformation of ethane

•  sigma  bonds  are  cylindrically  symmetrical  •  therefore, rotation is possible around C-C

bonds in open-chain molecules

Conformers

•  Conforma%on-­‐  different  arrangement  of  atoms  resul7ng  from  bond  rota4on  

•  Conforma7ons  can  be  represented  in  2  ways:  

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Torsional strain

•  We  do  not  observe  perfectly  free  rota7on  •  There  is  a  barrier  to  rota7on,  and  some  conformers  are  more  stable  than  others  •  staggered- most stable •  eclipsed- least stable

Potential energy versus bond rotation

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Conformations of other alkanes

•  The  eclipsed  conformer  of  propane  has  3  interac7ons:  two  ethane-­‐type  H-­‐H  interac7ons,  and  one  H-­‐CH3  interac7on  

Conformation of other alkanes

•  Conforma7onal  situa7on  is  more  complex  for  larger  alkanes  

•  Not  all  staggered  conforma7ons  have  the  same  energy,  and  not  all  eclipsed  conforma7on  have  the  same  energy  

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Conformation of butane

•  An7  conforma7on  •  methyl groups are 180° apart

•  Gauche  conforma7on  •  methyl groups are 60° apart

•  Which  is  more  stable  energe7cally?  

Steric strain

•  steric  strain  •  repulsive interaction occurring between atoms

that are forced closer together than their atomic radii allow

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Learning check

•  Draw  the  most  and  least  stable  Newman  projec7ons  (sight  along  the  C2-­‐C3  bond)  for  2,3-­‐dimethylbutane.  

Applications

•  We  need  petroleum  to  help  fuel  our  cars  •  Petroleum  deposits  are  thought  to  be  derrived  primarily  from  the  decomposi7on  of  7ny  single  –celled  marine  organisms  

•  Before  use,  crude  oil  must  undergo  frac7onal  dis7lla7on    to  separate  materials  based  on  boiling  point:  •  Straight-run gasoline 30-200°C •  Kerosene 175-300°C •  Diesel fuel 275-400°C

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Applications

•  Straight-­‐run  gasoline  is  a  poor  fuel  because  of  engine  knock,  an  uncontrolled  combus7on  that  can  occur  in  a  hot  engine.  

•  It  was  recognized  long  ago  that  straight-­‐chain  hydrocarbons  are  far  more  prone  to  inducing  engine  knock  than  highly  branched  compounds.  

•  The  higher  the  octane  number,  the  be_er  the  fuel.  •  The  final  product:  15%  C4-­‐C8  straight-­‐chain  alkanes,  25%-­‐40%  C4-­‐C10  branched-­‐chain  alkanes,  10%  cyclic  alkanes,  10%  straight-­‐chain  and  cyclic  alkenes,  and  25%  arenes  (aroma7cs)