The Chemistry of Everything Kimberley Waldron. Chapter topics

Transcription

1 The Chemistry of Everything Kimberley Waldron Chapter 3 Diamonds Carbon allotropes, covalent bonding and the structure of simple organic molecules Richard Jarman, College of DuPage 2007 Pearson Prentice Hall 1 Chapter topics Period 2 elements, the uniqueness principle, network solids Single, double, and triple bonds between carbon atoms Allotropes, bond energy, bond length Delocalized electrons, resonance Chemistry on a nanometer scale Electron dot structures, the octet rule, nonbonding electrons Drawing molecules: full structures, condensed structures, line structures VSEPR theory, three-dimensional shape in molecules Silicon chemistry 2007 Pearson Prentice Hall 2 1

2 Periodic table and element properties Members of the same group have similar properties The uniqueness principle: The second row elements are different from the heavier members 2007 Pearson Prentice Hall 3 Small size of carbon enables close approach Bonds to itself and other elements are strong Forms four bonds in tetrahedral arrangement diamond is one form of the element Carbon is special 2007 Pearson Prentice Hall 4 2

3 Diamonds are tough Each atom is bonded to 4 others Network solid extends indefinitely in all directions Robust tough material Reluctant to undergo reactions 2007 Pearson Prentice Hall 5 Carbon and covalent bonds Carbon has four valence electrons Formation of four bonds: C atom gains four electrons from other atoms Four bonds in carbon satisfy the octet - like a noble gas 2007 Pearson Prentice Hall 6 3

4 Diamond: more than a pretty face Diamond is chemically inert, hard High thermal conductivity makes it of interest in semiconductor chips Low stiction low tendency to stick or cause friction Miniaturized microelectronic devices 2007 Pearson Prentice Hall 7 Engineering the diamond surface Activate the surface of a diamond by adding small organic molecules Molecules can be used to tether other molecules to the surface Diamond can be a support for new chemical reactions 2007 Pearson Prentice Hall 8 4

5 Multiple bonds Close approach of carbon atoms permits formation of multiple bonds Double bond (4 electrons) Triple bond (6 electrons) 2007 Pearson Prentice Hall 9 Multiple bonds affect reactivity Carbon atoms with four atoms attached are saturated Carbon atoms with 3 or less atoms attached are unsaturated 2007 Pearson Prentice Hall 10 5

6 Addition reactions to double bonds Addition reactions add additional to the C atoms in a multiple bond Surface of diamond contains unsaturated carbon 2007 Pearson Prentice Hall 11 Graphite: another form of carbon Carbon exists as allotropes: different forms of the same element Graphite contains hexagonal sheets Diamond contains tetrahedral framework Graphite is slippery; diamond is hard 2007 Pearson Prentice Hall 12 6

7 Bond strength increases with bond order Bond length decreases with bond order Graphite sheets contain 4 bonds per carbon strong No bonds left over to hold individual sheets together slide easily over each other Bond strength and the properties of graphite 2007 Pearson Prentice Hall 13 Bonding types in carbon Different ways for carbon to satisfy bonding requirements Bonding to 4 atoms (all single) Bonding to 3 atoms (2 single + 1 double) Bonding to 2 atoms (1 single + 1 triple or 2 double) 2007 Pearson Prentice Hall 14 7

8 Bond strength and reactivity Bond strength depends on the type of atoms In traditional chemistry the weakest bond is broken first Tuning laser radiation to bond type can select target bond to break by matching the laser energy to the bond energy not the weakest bond 2007 Pearson Prentice Hall 15 Buckyballs: new allotropes Recently discovered allotrope of carbon contains 60 atoms Structures match familiar large objects Soccer balls contain 60 corners Buckerminster Fuller designed geodesic domes 2007 Pearson Prentice Hall 16 8

9 Poyhedron with pentagons and hexagons with 60 corners forms a closed symmetrical structure Each atom is bonded to 3 others Alternating single and double bonds provide stability Resonance Buckyballs up close 2007 Pearson Prentice Hall 17 Resonance and benzene Benzene contains alternating single and double bonds (average order 1.5) Two equivalent ways to draw structure Actual structure is average of two resonance hybrids Resonance structures are more stable 2007 Pearson Prentice Hall 18 9

10 Buckyballs and nanotubes Buckyballs have graphite sheets in spheres Nanotubes have graphite sheets in tiny cylinders Applications in displays, microelectronics, fuel cells Nanotubes can be aligned and manipulated at a very small scale 2007 Pearson Prentice Hall 19 Nanotanks Fuel cells use chemical reactions to produce electricity 2H 2 (g) + O 2 (g) = 2H 2 O(l) Environmentally friendly: No CO 2 or CO produced Nanotubes are potential hydrogen storage systems 2007 Pearson Prentice Hall 20 10

11 Electron bookkeeping: counting electrons in organic molecules Structure requires knowing where the electrons are Chemical reactions involve rearranging electrons Method for determining electronic structures 2007 Pearson Prentice Hall 21 Counting electrons 1. Count the total number of valence electrons In CH 4 C atom provides 4 electrons 4 H atoms provide 4 x 1 electrons Total number of electrons = = Pearson Prentice Hall 22 11

12 Determining the skeleton Determine how the atoms are joined in the molecule In CH 4 all the H atoms are joined to C The next stage is putting in the electrons 2007 Pearson Prentice Hall 23 Dot structures and the octet rule Each bond in the skeleton requires 2 electrons Add dots between the atoms to make bonds and complete octet round each atom (except H) 2007 Pearson Prentice Hall 24 12

13 How many bonds? Atoms form enough bonds to complete octets and pair up valence electrons Total number of electrons = Pearson Prentice Hall 25 More complex molecules Formic acid - HCO 2 H Total number of dots = 4(C) + 2(H) + 12(O) = 18 Complete skeleton uses 8 dots leaves 10 Distribute to complete octets for all atoms and make correct number of bonds for each atom 2007 Pearson Prentice Hall 26 13

14 Formula is C 6 H 10 Total number of dots = 6x4 + 10x1 = 34 Completion of bonds uses 32 dots Form C=C double bond uses 2 dots Lewis dot structure of cyclohexene 2007 Pearson Prentice Hall 27 Methyl groups are at the end of organic molecules 1 C atom, 3 H atoms Methylene groups are in the middle of chains 1 C atom, 2 H atoms Condensed structure conventions 2007 Pearson Prentice Hall 28 14

15 Toothpaste Toothpaste has several functions Addition of fluoride to strengthen teeth Abrasives to clean gums Other molecules are used for cleaning and for flavor 2007 Pearson Prentice Hall 29 Abbreviations for large molecules Large molecules like lauryl sulfate are difficult to write Condensed formulas make it easier 2007 Pearson Prentice Hall 30 15

16 Taking the letters out Line structures are more economical C atoms are shown as kinks in the line H atoms are not shown at all, but are assumed to complete the need for carbon to form 4 bonds 2007 Pearson Prentice Hall 31 The third dimension Lewis dot structures are 2-dimensional pictures and do not consider the real 3- dimensional shape Valence Shell Electron Pair Repulsion (VSEPR) theory takes Lewis dot structures and predicts 3-dimensional shapes Considerations: Electron groups repel each other Occupy positions of maximum separation 2007 Pearson Prentice Hall 32 16

17 Electron pair geometry and molecular geometry Electron pair geometry considers the bonded atoms and the nonbonded electron pairs Electron pair geometry of NH 3 is tetrahedral Molecular geometry considers only the bonded atoms Molecular shape of NH 3 is trigonal pyramidal 2007 Pearson Prentice Hall 33 These drawings are both correct dot structures for methylene chloride. Are they different? Three-dimensional views show they are the same there is only one structure for methylene chloride Vertex sensitivity 2007 Pearson Prentice Hall 34 17

18 Need to know electronic geometry Determining electronic geometry is necessary to find molecular shape NH 3 is a pyramid Considering only the atoms we will get the wrong shape 2007 Pearson Prentice Hall 35 The process for water 2007 Pearson Prentice Hall 36 18

19 The number of groups (atoms and lone pairs) determines electron geometry Ignoring the lone pairs gives the molecular geometry H 2 O and NH 3 have same electronic geometry Different molecular shape Many molecules, few shapes 2007 Pearson Prentice Hall 37 The number of bonds formed by atoms is easily predicted Carbon always forms 4 bonds 4 single bonds Tetrahedral geometry 2 single + 1 double Flat triangular geometry Patterns emerge 2007 Pearson Prentice Hall 38 19

20 VSEPR rules for organic molecules Same number of groups always gives same electron geometry 2 linear, 3 flat triangular, 4 tetrahedral Molecular geometry given by electronic geometry minus lone pairs 2007 Pearson Prentice Hall 39 Silicon: carbon s big sister Silicon has 4 valence electrons forms 4 bonds Si Si bonds are longer (and weaker) than C C bonds Silicon does not form wide range of chains and rings like carbon does Element silicon has same network structure as diamond 2007 Pearson Prentice Hall 40 20

21 Silicon the semiconductor In the silicon crystal all the electrons occupy the lower energy level Excitation of an electron into the upper level creates conductivity Increasing temperature increases number of electrons in upper level Silicon is a semiconductor - different from a metal 2007 Pearson Prentice Hall 41 Tweaking the conductivity Doping the semiconductor alters the conductivity by changing the number of charge carriers Dopants either have fewer electrons (Al) or more electrons (As) than Si Doping with Al creates holes Doping with As creates electrons 2007 Pearson Prentice Hall 42 21

22 Semiconductors and light Electrons can be excited into the upper level by absorption of light This is the basis of photovoltaic cells 2007 Pearson Prentice Hall 43 Semiconductors Dopants with fewer electrons create holes (p-type semiconductor) Dopants with more electrons create electrons (n-type semiconductor) Electronic devices are made by a combination of p-and n- type semiconductors 2007 Pearson Prentice Hall 44 22

23 Semiconductor devices allow control of electrons metal conductors do not The semiconductor diode is based on a p-n junction. Current flows one way but not the other Semiconductor devices form the basis of modern electronics The p-n junction can also emit light when a voltage is applied light-emitting diode (LED) Electronic widgets 2007 Pearson Prentice Hall 45 23