2. Organic Compounds: Alkanes and Cycloalkanes. Based on McMurry s Organic Chemistry, 6 th edition, Chapter 3

Transcription

1 2. Organic Compounds: Alkanes and Cycloalkanes Based on McMurry s Organic Chemistry, 6 th edition, Chapter 3

2 Families of Organic Compounds Organic compounds can be grouped into families by their common structural features We shall survey the nature of the compounds in a tour of the families in this course This chapter deals with alkanes, compounds that contain only carbons and hydrogens, all connected exclusively by single bonds 2

3 Functional Groups Functional group - a collection of atoms within a molecule with a characteristic chemical behavior The functional groups affect the reactions, structure, and physical properties of every compound in which they occur 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 3

4 Types of Functional Groups: 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 alternating single and double C-C bonds in a six-membered ring 4

5 Carbon Singly Bonded to an Electronegative Atom Alkyl halide: C bonded to halogen (C-X) Alcohol: C bonded to O of a hydroxyl group (C OH) Ether: Two C s bonded to the same O (C O C) Amine: C bonded to N (C N) Thiol: C bonded to SH group (C SH) Sulfide: Two C s bonded to same S (C S C) Bonds are polar, with partial positive charge on C ( +) and partial negative charge ( ) on electronegative atom 5

6 Carbon Singly Bonded to an Electronegative Atom 6

7 Carbon Oxygen Double Bond (Carbonyl Groups) Aldehyde: one hydrogen bonded to C=O Ketone: two C s bonded to the C=O Carboxylic acid: OH bonded to the C=O Ester: O bonded to the C=O Amide: N bonded to the C=O Acid chloride: Cl bonded to the C=O Carbonyl C has partial positive charge ( +) Carbonyl O has partial negative charge ( -). 7

8 Find the Functional Groups 8

9 Alkanes 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 C n H 2n+2 where the number of C s is n (n is any integer) They are also called aliphatic compounds or saturated hydrocarbons 9

10 Alkane Isomers CH 4 = methane, C 2 H 6 = ethane, C 3 H 8 = propane The molecular formula of an alkane with more than three carbons can give more than one structure C 4 (butane) = butane and isobutane C 5 (pentane) = pentane, 2-methylbutane, and 2,2- dimethylpropane Alkanes with C s connected to no more than 2 other C s are straight-chain or normal (n) alkanes Alkanes with one or more C s connected to 3 or 4 C s are branched-chain alkanes 10

11 Alkane Isomers: examples 11

12 Alkane Isomers 12

13 Constitutional Isomers Isomers that differ in how their atoms are arranged in chains are called constitutional isomers Compounds other than alkanes can be constitutional isomers of one another They must have the same molecular formula to be isomers 13

14 Drawing Chemical Structures Chemists use shorthand ways for writing structures Condensed structures: C-H and C-C and single bonds aren't shown but understood If C has 3 H s bonded to it, write CH 3 If C has 2 H s bonded to it, write CH 2 ; and so on. The compound called 2-methylbutane, for example, is written as follows: Horizontal bonds between carbons aren't shown in condensed structures the CH 3, CH 2, and CH units are simply understood, but vertical bonds are added for clarity 14

15 Skeletal Structures Minimum amount of information but unambiguous C s not shown, assumed to be at each intersection of two lines (bonds) and at end of each line H s bonded to C s aren't shown whatever number is needed will be there All atoms other than C and H are shown hexane cyclohexane two hydrogen atoms on each carbon Skeletal structure Condensed structure Skeletal structure 15

16 Skeletal to Condensed Structures: examples Example 1 Example 2 2 hydrogens here 1 hydrogen here 1 hydrogen here 3 hydrogens here Example 3 Example 4 1 hydrogen here 2 hydrogens on both carbons 16

17 Skeletal Structures: one example of natural compound 17

18 Molecular Models Drawings on paper and screens are limited in what they can present to you. We often need to visualize the shape or connections of a molecule in three dimensions Molecular models are three dimensional objects, on a human scale, that represent the aspects of interest of the molecule s structure (computer models also are possible) Framework models (ball-and-stick) are essential for seeing the relationships within and between molecules you should own a set Framework Space-filling models are better for examining Space-filling the crowding within a molecule 18

19 Names of small straight-chain Alkanes No. of C Formula Name (C n H 2n+2 ) Condensed structure 1 Methane CH 4 CH 4 2 Ethane C 2 H 6 CH 3 CH 3 3 Propane C 3 H 8 CH 3 CH 2 CH 3 4 Butane C 4 H 10 CH 3 CH 2 CH 2 CH 3 5 Pentane C 5 H 12 CH 3 CH 2 CH 2 CH 2 CH 3 6 Hexane C 6 H 14 CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 7 Heptane C 7 H 16 CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 8 Octane C 8 H 18 CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 9 Nonane C 9 H 20 CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 10 Decane C 10 H 22 CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 19

20 Alkyl Groups Alkyl group The partial structure that remains if a hydrogen atom is removed from an alkane (a part of a structure) General abbreviation R (for Radical, an incomplete species or the rest of the molecule) Name: replace -ane ending of the parent alkane with -yl ending CH 3 is methyl (from methane) CH 2 CH 3 is ethyl from ethane CH 3 CHCH 2 CH 3 CH 3 20

21 Alkyl Groups 21

22 Types of Alkyl groups Classified by the connection 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) 22

23 Alkyl Groups 23

24 Naming Alkanes: IUPAC nomenclature Compounds are given systematic names by a process that uses Prefix-Parent-Suffix Follows specific rules Named as longest possible chain Carbons in that chain are numbered in sequence Substituents are numbered at their point of attachment Compound name is one word (German style) Complex substituents are named as compounds would be 24

25 Naming Alkanes: IUPAC rules (1) Find the longest continuous carbon chain Choose the one with the larger number of branch points as the parent name. 25

26 Naming Alkanes: IUPAC rules (2) Number the atoms in the main chain starting at the end nearer the first branch point, and (3) identify and number the substituents If the first substituent has the same number starting from each end, number the atoms giving the lower number to the second substituent 26

27 Naming Alkanes: IUPAC rules

28 Naming Alkanes: IUPAC rules If two or more identical side chains are present, use the prefix di-, tri- and so forth. (4) Write the name as a single word, citing the substituents in alphabetic order. 28

29 Naming Alkanes: IUPAC rules Complex substituents are named using the same rules. Carbon 1 is the one bonded to the main chain. 29

30 Naming Alkanes: examples isopropyl-2,6,6-trimethylnonane (1-ethylpropyl)-2,6,6-trimethyldecane 30

31 Natural Occurrence of Alkanes Source: petroleum refining C 1 -C 4 : gases (natural gas) (<20 C) C 5 -C 8 : Gasoline ( C) C 9 -C 16 : Kerosene ( Diesel, ) C 17 -up: Lubrificating oil ( ) 2 CH 3 CH 2 CH 2 CH O 2 8 CO H 2 O 31

32 Properties of Alkanes Called paraffins (low affinity compounds) because they do not react as most chemicals They will burn in a flame, producing carbon dioxide, water, and heat (when used as a fuel in combustion) They react with Cl 2 in the presence of light to replace H s with Cl s (not controlled) 32

33 Physical Properties: boiling and melting points Boiling points and melting points increase as size of alkane increases Forces between molecules (temporary dipoles, dispersion) are weak 33

34 Physical Properties: solubility Similia similibus solvuntur Alkanes are not water soluble, they dissolve in non-polar solvents such as hexane, CCl 4, etc. 34

35 Cycloalkanes Cycloalkanes are alkanes that have carbon atoms that form a ring (called alicyclic compounds) Simple cycloalkanes rings of CH2 units, (CH2)n, or CnH2n Structure is shown as a regular polygon with the number of vertices equal to the number of C s (a projection of the actual structure) cyclopropane cyclobutane cyclopentane cyclohexane 35

36 Complex Cycloalkanes Naturally occurring materials contain cycloalkane structures Examples: chrysanthemic acid (cyclopropane), prostaglandins (cyclopentane), steroids (cyclohexanes and cyclopentane) 36

37 Properties of Cycloalkanes Melting points are affected by the shapes and the way that crystals pack so they do not change uniformly 37

38 Naming Cycloalkanes Count the number of carbon atoms in the ring and the number in the largest substituent chain. If the number of carbon atoms in the ring is equal to or greater than the number in the substituent, the compound is named as an alkyl-substituted cycloalkane For an alkyl- or halo-substituted cycloalkane, start at a point of attachment as C1 and number the substituents on the ring so that the second substituent has as low a number as possible. Number the substituents and write the name 38

39 Naming Cycloalkanes 39

40 Naming Cycloalkanes 40

41 End of chapter 2 41