Lecture 1 - Ulrich Hintermair
Terms : Hide Images
| 9551486108 | What are the features of the p block elements? | - Most p block elements can adopt more than one oxidation state. - Boron, (carbon), silicon, nitrogen and phosphorus are semi/non metals. - Form discrete molecules and polymers (rings and chains) that are generally poor conductors of electricity. - Down a group, there is more metallic character and larger radii that allow for higher coordination numbers. - First element of each group shows different chemistry due to (the number of electrons?). | 0 | |
| 9551486109 | What is catenation? | The linkage of atoms of the same element into longer chains. Catenation occurs most readily in carbon, which forms covalent bonds with other carbon atoms to form longer chains and structures. | 1 | |
| 9551486110 | What are the trends down the p block groups? | - Down the group, multiple single bonds between atoms of the same element are preferred to double bonds. - Down the group, there is an increasing s-p gap, so there is less favourable hybridisation. | 2 | |
| 9551486111 | What are the features of group 13 (3) elements? | ns2.np1, where n is the shell number. There are 3 valence electrons, so the maximum oxidation state is +3. Aluminium is the most abundant group 13 element. There is an increase in metallic character from boron to thallium. | 3 | |
| 9551486112 | What complexes do group 13 elements form? | They have 3 valence electrons. X=3 x 1e-. Total = 6e- = 3e- pairs, making trigonal planar complexes. - Around the element, there is an incomplete octet (only 6 electrons in the valence orbitals). - The 2Pz orbital is empty and low in energy (it's electron deficient). - Vacant orbital leads to Lewis acid behaviour, as it accepts an electron pair to complete the octet. - Forms 3 centre bonds; observe dimers, polymers and clusters with bridging groups. | 4 | |
| 9551486113 | What is a lewis acid? | A species capable of accepting a pair of electrons (acceptor), e.g. BX3, AlCl3, transition metals. | 5 | |
| 9551486114 | What is a lewis base? | A species with a pair of electrons available for donation (donor), e.g. H2O, NH3, F- and other halogens. | 6 | |
| 9551486115 | What are the features of boron? | - Boron is the only non metallic element in group 13, and forms covalent bonds. - Its chemistry is very different from that of the other elements that form covalent molecules. - Due to it having one less valence electron than number of valence orbitals. | 7 | |
| 9551486116 | What are the features of boron compounds? | - The boron partially overcomes the lack of electrons by forming p-p bonds. - These bonds occur between the halogen p orbitals and the empty 2Pz orbital on boron. - The relative order of ease of electron acceptance by BX3 is: BF3 < BCl3 < BBr3 < BI3. Iodine accepts the most electron density. | 8 | |
| 9551486117 | Why is BF3 a weak lewis acid? | One might think that as F is the most electronegative element that BF3 would be the most electron deficient molecule, but the F atom is of similar size to B, so forms a stronger p bond (as there's better overlap)m giving the most stable BX3 structure. Therefore, BF3 needs additional electron density the least, as is a weak lewis acid. BI3 is the strongest lewis acid in the series. | 9 | |
| 9551486118 | What are the features of boron hydrides (BH3)? | BH3 = borane. [BH4-] = tetrahydroborate anion. - Hydrogen atoms in BH3 don't have filled p orbitals able to stabilise the empty 2Pz orbital on B through p bonding. - BH3 is therefore a strong lewis acid - don't not exist as a discrete molecule. - BH3 dimerises to form B2H6 (diborane). This has a bridging H 3 centre-2 electron bonds. |  | 10 |
| 9551486119 | What are the features of alkali metal boronates? | NaBH4 and LiBH4. - Very useful in the lab as general reducing agents: they can donate electrons to other molecules (reducing them). Since it loses electrons, the alkali metal boronate is oxidised. - Both the alkali metal boronates and ammonia borane (NH3BH3) are potentially useful hydrogen storage materials. | 11 | |
| 9551486120 | What are the features of boric acid [B(OH)3]? | - Oxygen complexes of boron = borates, B-O bond is strong. - Simplest borate is boric acid, that forms trigonal planar layers. - It's a weak Bronsted acid and a Lewis acid: B(OH)3 + H20 <--> B(OH)4- + H3O+. - Used in organometallic catalysis to make new C-C bonds. - The borate anion, B(OH)4- is four coordinate and tetrahedral. - Borate ions (BO[x]^n-) can form trigonal planar or tetrahedral cyclic or linear polymers linked by bridging O atoms. | 12 | |
| 9551486121 | What are the features of boron oxide (B2O3)? | Prepared by dehydrating boric acid (heating it): 4B(OH)3 --> 2B2O3 + 6 H2O. - Ordered network of trigonal BO3 units joined through oxygen atoms. - Metal oxides dissolve in molten B2O3 to give coloured glass. - Boron oxide + silica = borosilicate glass (resistant to thermal shock - has low thermal expansivity of glass due to strong B-O bonds). Used to make heat-resistant lab glassware. | 13 | |
| 9551486122 | What are the features of BN compounds? | BN unit is isoelectronic to CC - similar structures to carbon. - Boron nitride (BN) has 2 structures (graphite like 2D layers and diamond like 3D layers). - Borazine (B3N3H6) has 6 p electrons in a delocalised ring structure - just like benzene. - Polarity of BN bonds make borazine more reactive than benzene. | 14 | |
| 9551486123 | What are the features of aluminium? | Al is essentially metallic. It's an electropositive metal, yet is inert due to passivating surface oxide film. - AlX3 is also planar and a Lewis acid. These are dimers in the gas phase. - Used as a Lewis acid catalyst for organic reactions including Friedel-Crafts alkylation reactions. | 15 | |
| 9551486124 | What are the features of thallium? | Thallium (I) is intensely poisonous. Its ionic radius is very similar to that of K+ ions. Therefore, it can enter cells and disrupt the mechanisms of K+ and Na+ transport. | 16 | |
| 9551486125 | What are the features of group 14 (4) elements? | ns2.np2. 4 valence electrons so the maximum oxidation state is +4. Straddle divide between metals and non-metals. | 17 | |
| 9551486126 | What is carbon like? | Non-metallic element, forms covalent compounds with other non-metals and structures with a high ionic character with electropositive metals. - Catenation is very important for carbon (the ability of an element to form covalent bonds with itself to give chains or rings). - Double bonds and triple bonds are more stable for C than other group 14 elements. | 18 | |
| 9551486127 | What are some small carbon molecules? | CO (triple bond carbon monoxide) and O=C=O (carbon dioxide). - Forms HC=-N and ionic cyanides containing the CN- ion. These are extremely toxic. | 19 | |
| 9551486128 | What is HCN like? | - Liquid with a faint bitter almond like odour. - Chemical warfare agent, first used in WWI. - Previously used in German extermination camps and in US executions. - CN- ions interfere with iron-containing respiratory enzymes. | 20 | |
| 9551486129 | What is silicone like? | - Forms some similar structures to carbon. - Both are strong oxophiles and flurophiles (both have high affinities for hard anions O2- and F-). - Silanes - very reactive alkanes that can spontaneously combust in air. - Silicone halides are more stable, and form tetrahedral molecules e.g. SiCl4, used to grow silicon surfaces for semiconductor manufacture. | 21 | |
| 9551486130 | What are silicon halides like (SiX4)? | - One or two Lewis bases (i.e. F-) can be added to SiX4 to form hypervalent (more than 8 valence electrons) 5 or 6 coordinate complexes, e.g. [SiF6]2-. - Silicon can expand its coodination number about 4 (unlike carbon). - Silicon is a good fluoride abstractor - used in different reactions. | 22 | |
| 9551486131 | Why can silicon be hypervalent? | - Stems from silicon being able to access low lying d orbitals (e.g. (SiH3)3N = trisilylamine, trigonal planar, non-basic. - Has multiple N (pp) --> Si (dp) interactions, between filled N Pz orbital and empty Si d orbitals. - Carbon analogue is tetrahedral (sp3) and basic. | 23 | |
| 9551486132 | What is the difference in reactivity in Si and C? | Si compounds can undergo reactions via nucleophilic substitution mechanisms (SN2). Their ability occurs much faster than carbon. | 24 | |
| 9551486133 | What are the features of silicon oxides? | Si forms multiple Si-O, as Si has a high affinity for oxygen. No Si=O bond, forms silicon polymers with rings or chains having single Si-O bonds. | 25 | |
| 9551486134 | What are the features of silicates? | Silicon-oxygen compounds form a vast variety of extended arrays derived from [SiO4]4- units. Silicate structures confined to tetrahedral, 4 coordinate Si. These crystallise very slowly. | 26 | |
| 9551486135 | What are zeolites? | Silica structures with the Si replaced by Al atoms (aluminosilicates) to form pore-like structures that can trap molecules. These zeolites have a wide range of applications, e.g. water softening, purification, removal of gases and catalysis. | 27 | |
| 9551486136 | What can zeolites do? | Zeolites absorb molecules that are smaller than the aperture dimensions (e.g. H2O, CO2). - Can also exchange ions for those in a surrounding solution (e.g. exchange Na+ for Mg+ - size exclusion). - The controlled size of the pores also means that host-quest controlled asymmetric synthesis is possible - only a certain shape of molecule is allowed to be formed inside the zeolite. | 28 | |
| 9551486137 | What is a semi conductor? | A substance that can conduct electricity under some conditions but not others (this is a good medium for the control of an electrical current). | 29 | |
| 9551486138 | How is Si semi-conductive? | Si has a band gap, i.e. a narrow forbidden energy range where no electron states exist between the valence band and the conduction band. - Overlapping bands make conductors. - Small band gaps make semi conductors. - Large band gaps make insulators. If electrons can transfer from the valence to conduction band, then a current can flow. | 30 | |
| 9551486139 | What are the applications for Si? | Integrated circuits, computer chips, solar cells. | 31 | |
| 9551486140 | What are the features of Tin, Sn? | - Resistant to corrosion. - Bronze is an alloy of Cu and Sn (<12%). - Solder is an alloy of Sn and Pb. - Trialkyl and triaryltin compounds (stannanes) are mild transmetalation agents useful in organometallic chemistry and catalysis, e.g. Pd-catalysed stille coupling. | 32 | |
| 9551486141 | What are the features of lead, Pb? | - Exception of +4 oxidation state in group 14 is lead, where most common oxidation state is +2 (inert pair effect). - Pb2+ forms more stable compounds with soft anions (e.g. I- and S2-) than with hard anions. - Alkyl lead compounds are highly toxic. - Pb is malleable - used in plumbing and solder. Now illegal due to lead poisoning concerns as it interferes with some body processes (e.g. development of the nervous system in kids). | 33 |
