Carbon Compounds. Chemical Bonding Part 2

Transcription

1 Carbon Compounds Chemical Bonding Part 2

2 Introduction to Functional Groups: Alkanes! Alkanes Compounds that contain only carbons and hydrogens, with no double or triple bonds.! Alkyl Groups A part of a compound that contain only carbons and hydrogens, with no double or triple bonds. These and others can be designated by R

3 Introduction to Functional Groups: Alkanes! Alkene Carbon-based compound that contains at least one double bond.! Alkyne Carbon-based compound that contains at least one triple bond.

4 Phenyl & Benzyl Groups! Phenyl group benzene ring substituent! Benzyl group benzene ring with a -CH 2 - bridge

5 Alkyl Halides and Degree of Substitution! Alkyl halide (Haloalkane) An alkane with a halide (Cl, F, Br, I) as a substituent.! Degrees of Substitution:! Primary (1 ): carbon with the attached halide has only one other carbon attached to it.! Secondary (2 ): carbon with the attached halide has only two other carbons attached to it.! Tertiary (3 ): carbon with the attached halide has only three other carbons attached to it.

6 Other Functional Groups! Alcohols! Ethers! Amines! Carbonyls: Aldehydes Ketones Carboxylic acids Esters Amides Acid chloride! Nitriles! Thiols! Sulfides

7 Summary of Important Families of Organic Compounds

8 Summary of Important Families of Organic Compounds

9 Ionic and Covalent Bonds! An ionic bond is a bond in which a cation and an anion have an electrostatic attraction for one another.! A covalent bond is a bond in which two atoms share electrons. It is the mutual attraction of two nuclei for the same electrons that keeps the atoms together.

10 Bond Polarity! In reality, few bonds are completely ionic or completely covalent.! The electronegativity difference (ΔEN) between two atoms will allow us determine whether two atoms in a bond will display more ionic properties or more covalent properties.

11 Bond Polarity! If the electrons are shared evenly, the bond is nonpolar.! If the electrons are shared unevenly, the bond is considered polar it has a positive side and a negative side.! If one atom has an attraction so much stronger than the other atom that it pulls the electrons away, then we consider the bond to be ionic.

12 Bond Polarity There is a continuum from non-polar to ionic, with no clear breaks. However, we will define some arbitrary cut-off points: ΔEN 0.4 Nonpolar Covalent Bonds (electrons evenly shared) 0.5 ΔEN 1.6 Polar Covalent Bonds (electrons unevenly shared) ΔEN 1.7 Ionic Bonds (full charges)

13 Electronegativity Chart

14 Molecular Polarity! Bonds are considered polar or non-polar based on electronegativity differences.! A molecule will only be a dipole (polar) when 2 criteria are met: 1. There is a polar bond in the molecule. 2. There is asymmetry with respect to charge.

15 HCl & dipole moment! Consider HCl:! The dipole moment of a molecule can be measured experimentally! It is the product of the magnitude of the charges (in electrostatic units: esu) and the distance between the charges (in cm).! The actual unit of measurement is a Debye (D) which is equivalent to 1 x esu cm

16 Map of Electrostatic Potential (MEP)! A map of electrostatic potential (MEP) is a way to visualize distribution of charge in a molecule.! Parts of the molecule which are red have relatively more electron density or are negative! Parts of the molecule which are blue have relatively less electron density or are positive! The overall shape of the molecule and van der Waal s radii are depicted is also represented.

17 Methane (CH 4 )! Are the bonds polar? ΔEN = = 0.4 à NO! Therefore, methane is a NON-POLAR Molecule.! We do NOT need to ask the second question: Is there any charge asymmetry in the molecule?

18 Ammonia (NH 3 )! Are the bonds polar? ΔEN = = 0.9 à YES, polar bonds:! Is there any charge asymmetry? δ N H δ + Consider the shape: Trigonal pyramidal à YES! Therefore, Ammonia is a POLAR Molecule.

19 Water (H 2 O)! Are the bonds polar? ΔEN = = 1.4 à YES, polar bonds:! Is there any charge asymmetry? Consider the shape: Bent (~105 ) à YES! Therefore, Water is a POLAR Molecule. δ O H δ +

20 MEP Water and Ammonia! An unshared pair of electrons on atoms such as oxygen and nitrogen contribute a great deal to a dipole.! Water and ammonia have very large net dipoles.

21 Carbon Dioxide (CO 2 )! Are the bonds polar? ΔEN = = 1.0 à YES, polar bonds:! Is there any charge asymmetry? Consider the shape: Linear à NO δ + C O δ! Therefore, carbon dioxide is a NON-POLAR Molecule.

22 Boron Trifluoride (BF 3 )! Are the bonds polar? ΔEN = = 2.0 à YES, polar bonds: (despite large ΔEN, bonds have covalent character) δ + B F δ! Is there any charge asymmetry? Consider the shape: Trigonal Planar à NO! Therefore, BF 3 is a NON-POLAR Molecule.

23 Formaldehyde! Consider CH 2 O.! CH 2 O is a POLAR molecule.

24 Carbon Tetrachloride and Chloromethane! In carbon tetrachloride the bond dipoles cancel and the overall molecular dipole is zero Debye! In chloromethane the C-H bonds have only small dipoles but the C- Cl bond has a large dipole and the molecule is quite polar.

25 Other molecules! Consider COCl 2.! COCl 2 is a POLAR molecule.! Consider CH 2 Cl 2.! CH 2 Cl 2 is a POLAR molecule.

26 Dipole Moments of some molecules

27 Molecules with more than one central atom! Consider the polarity of bonds in CH 3 CN.! CH 3 CN is a POLAR molecule.

28 Isopropyl Alcohol! Consider isopropyl alcohol: CH 3 CH(OH)CH 3.! CH 3 CH(OH)CH 3 is a POLAR molecule.

29 Dipole moments & cis-trans isomers! Some cis-trans isomers differ markedly in their dipole moment.! In trans 1,2-dichloroethene the two carbon-chlorine dipoles cancel out and the molecular dipole is 0 Debye! In the cis isomer the carbon-chlorine dipoles reinforce and there is a large molecular dipole

30 Longer Chain Molecules! Consider the molecule below: OH! Is it a polar molecule?! Yes, but it also has a significant non-polar portion. This will significantly affect its solubility properties.